I'm a strait man, happily married. My wife thinks I'm handsome as a man, and can't keep her hands off me when I'm dressed as a woman.
I love women so much (perhaps too much) that I need to do more than just look at them - I like to see myself as one, dress like one, feel what it's like to be made up as one, etc.
I'm not even a smidgen gay or bi. If I woke up tomorrow morning as a
real girl, I'd be a lipstick lesbian!
In terms of why are some crossdressers strait, some bi, and some gay? I think if you replace "crossdressers" with "people" in my last sentence, it spells out my personal theory on it. What does it matter that we like to dress and look like women? There are all kind of people in this world. Why would the crossdressing community be any less diverse than a community of people who happen to like peanut butter?