I’m a straight girl and found my roommate on Facebook. My roommate told me ahead of time that she is bisexual, since sexuality IS something that comes up in college and she wouldn’t want me to be uncomfortable. I’m not uncomfortable at all with it and, in fact, the majority of my female friends happen to be lesbian/bi, but I can see that some people might be.
It’s not clear to me whether the OP is planning to be ‘out’ in college or not. If he is, then I think it’s obvious that the roommate should/would know. If not, it all depends on comfort level. Maybe gauge how socially liberal this roommate is and decide whether or not to tell her from there. I imagine that the roommate might be a bit taken-aback or surprised, but hopefully she would have no problem with it.
Some colleges are starting to offer Gender-Neutral Housing. You might want to look into that.