Here's another thread with a similar theme: http://www.wrongplanet.net/postt194436.html
I'm in a different situation, because I've been in the same hetero relationship for 23 years. But, as ThehouseholdCat says, maybe the reason I picked a man was due to my upbringing (although my parents are pretty liberal minded these days). I've had a brief same sex relationship, before I met my husband. I was just 16 then, so I was very young. These days, I'm attracted to my husband only, no-one else. I don't look at other people in that way at all. I'm more likely to look at a woman's body than a man's body, for the aesthetic pleasure (although my husband tops anyone else on that front, in my mind). However, I think that's the artist in me, not attraction (except for in the case of my husband, where it is definitely attraction). I also find people who are somewhere close to the imaginary boundary of the genders to be intriguing and desirable. In any case, I've no intention of ever acting on any desires (if that's what they are). I'm pretty certain that there's a chance I'd enter into a same sex relationship, if my marriage ever ended (which I really hope doesn't happen).
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"We act as though comfort and luxury were the chief requirements of life, when all we need to make us really happy is something to be enthusiatic about." Charles Kingsley