Simon Levay (biological scientist) says,
Although there are significant differences between the attitudes of lesbians and gay men it is clear that both groups are far more inclined to consider their sexual orientation a biological given than is the general population....Should we take these assertions seriously? Not entirely, of course. No one even remembers being born, let alone being born gay or straight. When a gay man, for example, says he was born gay he generally means that he felt different from other boys at the earliest age he can remember. Sometimes the difference involved sexual feelings, but more commonly it involved some kind of gender nonconformist or sex atypical traits-disliking rough and tumble play for example, that were not explicitly sexual. These differences, which have been verified in a number of ways suggest that sexual orientation is influenced by factors operating very early in life, but these factors could still consist of environmental factors such as parental treatment in the early postnatal period.
LeVay, Queer Science, 6.
You got so many varying opinions on this that when it comes down to the scientific method, there isn't anything conclusive. I know the APA has quite a few articles on it, but also speaks of it from more of a psychological perspective.