In graduate school I became friends with a gay guy in astrophysics; the year I finished, another gay astro student arrived. So there are gay scientists out there! (and - good news for rbot - a lot of them seem to be in astro!)
My friend is now married to a guy with a background in an arts discipline and computer science.... In any relationship - gay or straight - I think that people want their partners to value their interests and (especially in the sciences, where you never really stop thinking about what you're doing at work) understand their motivations and frustrations. I think this is why half of married female physicists are married to other physicists! I also think it's seen as more acceptable for men to keep their work lives out of their marriages, whereas women want their husbands to encourage and support them in their work.
Since the topic is on the table, I thought I'd share an interesting blog entry by a gay mathematics grad student who discusses the pros and cons of being out to the students he teaches: