A couple other comments here have said the same thing, and I strongly believe this is the actual reason. Queer people flock to cities to improve their dating pool and social acceptance. College is a legitimate way to move to a city (and meet other gay peers). Source: have a fair amount of gay friends.

On the other hand, if you already live in a liberal metropolitan area, it's potentially more likely that more of your peers go to college so you go too (no source here, just spitballin).