It's easy to assume that. It really is a very rough guide though, so if you stick to it, don't be surprised if you end up looking like a complete idiot on several occasions throughout your life.
Similar issue: I worked with someone via email for several months thinking they were female because their name is Kim before a coworker mentioned "uh, pretty sure Kim's a dude." You run into similar issues with e.g., women named Chris. I don't think being thrown by situations like that makes anyone look like an idiot.
Looking at it from an ethnic perspective, I'd be similarly thrown by finding out a guy named Santosh was a European white male.
Ideal world, sure, we wouldn't make assumptions about identity based on names. But it happens. I'm not sure that there is an answer that'd make everyone happy, to be honest.
Of course we would, unless in the ideal world names would be uniformly distributed among population in a completely random way. But they're not.
The only problem here is seeing reasonable inference from a known probability distribution as ethically dubious.
If you tend to make a lot of assumptions about someone just from their name before encountering them, then you are pretty much guaranteeing that you are going to get burnt a lot over time, unless you don't meet many people, as it simply isn't that good a guide.
For me when I'm mistaken, it's just a matter of making note of it and moving on. Not a big deal.
That's essentially the same point I was getting at.