First, your friend is totally awesome.
That being said, I think there are many possibilities as for why housing is segregated by sex.
1. Tradition. No one complained in the past, so no one should complain now...it's how it's always been. Obviously, this isn't the case, but it's how it may be viewed.
2. Pregnancy. No one wants to send little Suzy off to college only to get her back for summer break with a big ol' pregnant belly. Is it the college's fault? Of course not, but parents will sure as hell try to sue in protest. After all, heaven forbid they look at little Suzy and say, "Well, you had sex and now this has happened. So what's your plan?" It's much easier to be pissed off and blame an entity that isn't responsible.
3. Fear of rape. This one is a stupid argument because women rape women and men rape men, and people will rape strangers so it's not like this will prevent it but I'm sure it's an argument used.
4. Some people are uncomfortable around the opposite sex. Some people are still raised that being around the opposite sex is really only something you're "allowed" to do in groups or if you're married. I was raised that way. Even if the person doesn't feel this is right, that person will still have been segregated from the opposite sex so much that it will be a culture shock. Well, WELCOME TO FUCKING COLLEGE, it's supposed to be a new experience where you learn new shit.
I think the only one that holds any clout would probably be number four since you don't want people living uncomfortably...in which case, they should be allowed to put down the gender of the person they wish to live with. Something like, "Check the box(es) of the gender you prefer to live with:
__ Other (transgender, genderqueer, etc.)"
This way, they can appeal to ALL students.
I'll respond to anything but am partial to chief.
"Life hands everyone shit. It's what you choose to do with that shit that determines if you're happy."
"Complacency is a shitty reason to be miserable."