Alrighty! Differences I found between homosexual and heterosexual people:
1. Erotic material that is of opposite sexual orientation for both not only doesn’t arouse, but it seems to cause an intense automated response of “that’s gross I don’t want to look at it” that can be seen in the nervous system. This isn’t really a difference between the two, but it’s interesting nonetheless.
2. The corpus callosum is the structure in the brain which connects the left and right hemispheres. In homosexuals, a particular part of it is larger than in heterosexuals. The corpus callosum is 94% determined by heredity.
3. Homosexuals are more likely to be left handed or ambidextrous than heterosexuals.
4. Testosterone (the male sex hormone) levels in the womb are higher for homosexuals of both gender than heterosexuals.
5. One study gives evidence for longer and thicker penises in homosexuals compared to heterosexuals.
6. Points four and five suggest that homosexuals are over-masculinised from before birth, which was somewhat surprising to me because of the popular notions of gay people being 'girly'. Popular opinion is wrong yet again!
7. Studies using animals found that deliberately damaging sections of the hypothalamus (which, among other things, links the hormonal system to the nervous system) causes a change in sexual preference behaviour.
8. I’m sure most of you know that in general men are better at maths and reading maps, but worse at verbal tasks than women. In homosexual men, it seems that they are more ‘female’ in this sense, which suggests a difference in brain organisation and also provides evidence for point two. Lesbians do not show any such difference in these kinds of tasks.
9. Studies with mice suggest that there are differences in the way oestrogen (the female sex hormone) is used, both in the womb and after.
10. There is minimal evidence for parenting being the cause of homosexuality – 90% of sons of gay fathers are heterosexual. There also isn't a lot of empirical evidence for life experiences being a factor in sexual orientation, but there haven't been many studies on it.
Overall, it's not really known what 'causes' homosexuality (or bisexuality for that matter). Sexuality is a complicated thing, there's a lot of things that could be factors. Also science still doesn't know everything about the brain, which makes it difficult to pinpoint exact reasons.
I think those are the main things. I was afraid of going overboard and writing too much, and I think I've included things that are irrelevant. Hopefully I've explained it all properly and I haven't bored anyone out of their skulls. Also, if I haven’t explained anything clearly enough, just say and I’ll try explaining it properly.