One way for this to come up is by mentioning activities-a PNM could say she is involved with an LGBT group on campus, and this is a good way to bring up sexuality and gauge actives' reactions without having to say "So I'm gay, is that a problem?"
I am a very open and accepting person (our chapter has several gay women, even one pair who is dating) but I would be a little put off if a PNM just brought up her sexuality out of the blue. Another tactic that could work is to talk to a sister you know- either one you have known or a woman you feel you've built a relationship with during rush- and confide in her. Say "I know this is a little awkward, but I really want to make sure XYZ would be ok with taking a member who is gay..."
Just my 2 cents on a really old thread

but maybe it'll help someone in the future