Ahem...
The reason you (mankind) are attracted to the opposite sex is because as a race of mortal beings, reproduction needs to occur. To reproduce you need a man and a female... but why reproduce? because you like having sexy time... thats it.
The thing that attracts you to the opposite sex is your hormones, the dominant hormone responsible for this in males is testosterone, it makes you attracted to females and is what makes you interested in other manly things (lol wording) i.e.: sports, guns etc
In the female, this hormone is estrogen (sp?) it attracts her to males and is responsible for women's interest in womenly things nail polish, shoes etc
Any way... have you noticed how gay men have a tendency to act 'girly'? not all but a good bunch of them. this is because they have an excess of estrogen, which is making them attracted to MALES, as well as like the girly stuffz
on the other end of the spectrum, homosexual women are usually quite manly and this is for the exact same reason... they have an excess of testosterone
hope i helped

Edit: yes this excess in hormones is present at birth

i did not pick an option in the poll however, because environment DOES have a minor effect on the outcome, because brainwashing through repitition is possible