JL started YC with PG (and other two letter humans) and that seemed to work out ok.
If you're in a significant relationship, that person is your partner; major life decisions surrounding employment, business, balance between home & office, etc. etc. aren't made in a vacuum.
Consider also that in some jurisdictions, your partner may be entitled to a share of the company as part of a divorce or similar proceedings (happened to a colleague of a friend here in Australia).
Also speaking from experience: your significant other will likely be an invaluable source of support, motivation, opinion, experience, etc.
Since then we've launched a successful product (http://grabaperch.com) allowing us to move away from doing consultancy work, our second product launched yesterday.
It can be tough, we talk about work all of the time. However there are good points. We decided pretty much on a whim to relocate to somewhere we fancied living - as the company is just us two permanently we could do that.
It is a great thing to feel that as a partnership in all senses we are building our future together. The flip side of that is that when things are not going well we tend to end up going round in circles a bit as we only have each other to talk to on a work and personal level.
I don't know that I'd recommend it - but I love my life, and this is how I got here.
Also, think about it this way: if you fail, there is a high likelyhood that you both will be feeling down, blaming each others shortcomings for the collective failure, and be stretched with no money and no jobs between the two of you. A highly stressful situation that only the best relationships will survive. Are you ready for that? The "If you start a business with your spouse, you're going to have a bad time" meme comes to mind here.
That is simply false, based on direct personal experience, and observations of other people close to me.
For example, I know a guy who co-founded a company. His co-founder got divorced, and his wife (not involved in the company in any way) got a slice of the company as partial divorce settlement.
My own experience was co-founding a company where my wife was not, on paper, 'involved'. But she was an invaluable source of support, advice and motivation.
Perhaps to ask this question differently: how do you think it's possible to have a significant relationship with someone, while completely excluding them from the other most important endeavour in your life?