Yeah well, societal pressures only go so far in influencing a relationship. If the other party can't respect your values, an argument over a diamond is the least of your worries.
It's very easy to justify going the no-diamond route, sure. From a financial point of view, it's just a bad idea. Some say the finances aren't the point, so you can turn to ethical arguments (diamonds are bloody business). You can't really lose the no-diamond argument; it's too easy to make.
Still, it's awfully convenient for the guy to be the one making the argument against buying an expensive ring. This doesn't make the arguments wrong, but depending on the relationship, the girl might suspect that the guy is making those arguments, not out of financial responsibility or morality, but out of apathy or simply being cheap. The guy might be making sound arguments, but his ulterior motive might still be problematic, and the girl might be taking issue with that. Also, there is a whole lot of meaning (artificial, but real just the same) that goes into the rituals of marriage, and you can't just dismiss all of that with some textbook argument about diamond cartels.
The relationship isn't doomed in this case; the couple can grow past it, and it's an opportunity to understand each other. The problem is, if either the guy or the girl takes on the perspective of "I'm obviously right and you're wrong if you disagree," then they're being unreasonable and stalling out the relationship. Both parties need to be honest and understand the other to get past it. The discussion over the diamonds may in fact be very important because it gets to the heart of a bunch of concurrent issues.
(I'm using "guy" and "girl" here because that's the situation where these cultural norms are at play. Obviously any other combination of genders could have the same issues, though I suspect it's less common in those cases.)
Going with the nuts advice of spending a month's salary (or more!), yeah, that is awful, but, as I did, you can buy a diamond ring for less than < 1k (Canadian!). I'd have preferred not buy anything, but as you said, being reasonable in your relationship is about understanding what your partner values as well and try to reach a middle ground.
You are conflating moral values with financial value.
If it was about "expressed commitment" donating a significant sum of money to a charity would be equivalent to buying a diamond.
Now I wonder if that would float...
For example, the actual act of popping the question is an important part of the tradition, and it's inextricably linked to the ring. We all know how it goes: the guy and the girl are on some sort of date, the guy says something vague, he pulls out a small box and opens it, and the girl can see the ring inside. She knows what this means before he even says anything, and is already nodding her head (hopefully) before he asks, "Will you marry me?"
That's all very easy to follow. Now imagine the guy pulls out the keys to a house or something instead of the ring. She wouldn't realize what's happening until he actually asks the question, which is fine of course... but it's not the same tradition, and it might not have the same magic to it. Beyond the proposal itself, the ring is a physical difference that she will carry through to the next day, which will prompt excitement from her friends and coworkers and remind her of that moment.
And that's just one element of the whole tradition. I think it's very easy to see why one would value the diamond. I just said that the price was about expressed commitment. There's much more to the tradition than that.
A woman who's really an equal partner would rather see her husband do something useful with that money, such as make a down payment on a house.
The whole concept of spending 2 months' salary on a diamond ring is the product of the most successful marketing campaign in history, by the deBeers company back in the first half of the 20th century. It is not something steeped in tradition (or at least nothing that goes back that far, or is the product of anything besides corporate marketing). So someone who insists on a real diamond ring is merely a dumb stooge who's easily swayed by advertising if you think about it.
Second, the fact that the tradition is a modern one, that started with an advertising campaign, is basically irrelevant. As far as the woman is concerned, this is something that every other married girl she's ever known has received. It's perfectly natural to want hers too. If she's particularly financially minded or happens to be exceptionally grounded in this case, she might decide to reject the tradition, but it's hardly a marker of unintelligence to feel otherwise. I'm sure you hold many irrational preferences in your life, as do we all, and it's not any more fair to insult her for her wishes than it is to insult you for yours.
I find it interesting that you suggest a down payment on a house as an alternative to buying a diamond ring. First, you're presuming that the diamond ring is such a burden that it's getting in the way of the couple's other financial obligations (which may be the case, but also may not). But also, your suggestion proves just how subjective and emotional these things really are, because property ownership is yet another optional part of "the dream" that many might consider a needless extravagance, much like a diamond ring. Sure, it seems to have more utility (on the surface), but it's also far more expensive, and it will continue to become more expensive as time passes on. Property ownership is valued much more highly in some cultures than others, and the notion of saddling yourself with debt (a mortgage) for 30 years just for that privilege seems downright insane to some people.
Is this dehumanizing? Damn right it is. If you demand a dehumanized relationship, then of course it's dehumanizing. If you don't want to be dehumanized, then demand a relationship based on equality. I'm not going to treat a woman as an equal if she doesn't act like it, and instead demands to be treated as some kind of possession, which is the only reason there is to spend 3 month's salary on a stupid rock for her.
And yes, it absolutely IS a marker of unintelligence for her to not reject this idiotic tradition. What do you think of women who demand to have their or their daughters' genitals circumcized because of tradition? That's pretty stupid too. This is no different. Again, if you want to be treated as an intelligent person, then start acting like it. There is no rational basis for wanting a diamond ring worth 3 months' salary. And while I may not be perfect as far as rational preferences, I try damn hard to make sure I am rational and am not clinging onto stupid beliefs for no good reason. This also isn't to say that all traditions are stupid: for instance, I like the tradition of putting up a Christmas tree (which of course comes from Germanic pagan traditions). It looks nice, it's seasonal, it's fun putting up the ornaments with family, etc. And these days you can get artificial trees so you're not killing trees for the tradition (you can also get trees with intact root balls and then replant them after the holidays). But the important key here is cost versus benefit: holiday traditions are fun and a nice family practice, but they're also pretty cheap. Artificial trees and ornaments are not at all expensive these days, unless you're desperately poor.
Now, as for your comment about a ring being a burden, it damn well is a huge burden, for everyone. Did you forget the part about it having to be 3 months' salary? This isn't like a Christmas tree which is generally fixed-price, it scales to whatever your income is. Can you seriously afford to take 3 months of your salary, withdraw it as cash, and set it on fire? I don't know anyone who would do anything that stupid; if I'm going to lose 3 months' pay, I might as well just take a 3-month unpaid vacation instead of working my ass off for money when then gets wasted.
And no, a house downpayment is not a "needless extravagance"; that is an utterly stupid thing to claim. People need a place to live, and while you can debate things like rent vs. buy, real estate appreciation rates, and the current market conditions, in general, for middle-class people, home ownership in the US is a fairly stable store of value if you don't plan to move for a while, and owning your own home means you don't have to pay rent any more. If you don't like the mortgage example, change it to something else, like putting the money in the bank or a stable value fund in case you lose your job. Or into a good index fund, where it's most likely going to appreciate significantly over the years. There's plenty of places where you can invest 3 months' pay that are perfectly sensible and will give you a far better return than just lighting it on fire.
There are many things people "waste" money on, yet they don't deserve to be called sub-human or property. People pay thousands of dollars extra for luxury car models. People pay millions of dollars for art that they can't appreciate any more than a dollar store painting by Joe-painter. People pay hundreds of dollars for expensive meals that contribute to the suffering of animals. These are the extreme cases, but every human being that has expendable income will buy pointless things simply because they like them. Heck, weddings themselves are usually more expensive than the ring, and they're an even greater waste of money; at least you can sell the ring for something later on. People pay for memories and for experiences (which the ring also provides, by the way); compared to those things, which hold zero financial value long-term, overpriced gemstones are actually a relatively good investment. It's more like buying an overly huge TV than setting the money on fire.
Yet once again, even if they were the worst investment in the world, your pride in taking a dehumanizing stance is bothersome.
I don't know why you would think the woman is contributing nothing to the relationship. Why not assume the man is contributing nothing? Maybe the woman is the breadwinner and highly supportive while the man pursues some artistic dream that generates little income? Forget the finances; why can't a woman contribute mentally and emotionally to the relationship? I simply can't understand where your comments are coming from.
The most troubling part of the tradition is, of course, that the man is expected to do it. Except the woman is also expected to receive it. If you personally refuse to buy a ring, then you need to find a partner who agrees with your stance. But, even a woman who doesn't care that much might still be put into uncomfortable situations (by friends, family, etc.) by not having one. To her, the social inclusion might mean she would prefer to have one (plus, it looks nice). If your partner makes it clear to you that they value a ring, then there's no foul play; it's simply one more part of their complex personality. It's simply an emotional preference; intelligence doesn't come into it. You can choose whether that's a deal breaker for you. Personally, I'm happy to buy the ring, because I enjoy buying nice-looking things for other people, and the ring is one I'll get to appreciate every day I see it. Like having children or living in Hawaii, it's something that one person might value more than the other, and it's important that the couple come to an understanding before they commit to each other.
Your fixation on the 3-months salary is misguided. First off, that's a guideline, not a requirement (I've heard of few brides who count the pennies, and the ones that do make their values quite obvious). Second, I seriously can afford to spend 3 months of my salary on a luxury item. I understand and sympathize that not everyone is as comfortable a financial situation as I am, but just like every single purchase, how much of a "burden" it is to you depends entirely on your financial situation.