Okay, for all you people who don't know what's up with sex & orientation, I'm here to explain it to you, in the form of easy bullet points. These are proven scientific facts, or possibly just my opinions, but don't worry about that part -- just relax and enjoy the benefits of my great wisdom, revealed here for you.
* Human sexual desire is an extremely strong urge, and it can attach itself to any number of things, including but not limited to silly costumes, pointy-heeled boots, animals, children, dead people, and rubber chickens.
* Human sexual orientation can be thought of as a position on a spectrum: at one end of the spectrum are people who only and always feel attraction for men. At the other end of the spectrum are people who only and always feel attraction for women.
* People of either gender are found across the spectrum.
* Most males are down near the female-only end, and vice-versa. Which is good because it makes babies happen.
* Some people are in the middle -- they can bat either way, depending on who's pitching and whether there are runners in scoring position.
* Most straight people think they live at the absolute end of the spectrum, but they really don't. Given the right circumstances (or formative experiences), most people could probably learn to respond sexually to their own gender. Not that we're advocating that, just saying.
* A certain percentage of women who are with women are there not so much because of biology as because
a) men have treated them so crappy, or
b) as a philosophical/gender-political statement or position;
Other women are with women because they just prefer them.
* Some men are with men because they just don't get how to be with women, but that percentage is very small compared to the equivalent women-with-women segment.
* Some people know from the time they're very small which gender they're attracted to; it's not a deliberate choice they made.
* Some people develop an attraction for their own gender in their quest for new and varied experiences.
* Sexual experiences when we're very young can have a formative effect -- a young boy whose first experience is with a man (ie, who's abused) has a much higher likelihood of being a gay adult than a young boy who doesn't have that experience.
* People who are sexually abused tend to inappropriately sexaulize other relationships later in life -- they often have trouble with boundaries, and they view sexuality as a natural extension of affection, even in circumstances when society says it isn't.
Any other questions, just ask...