I believe it's both. You see many cases where it's just the way a person grows up that causes them to be attracted to the same sex.
But, I believe more so in nature just from what I've seen in my own family.
My grandmother had seven children (
) including my father. His oldest brother had always felt since he was little that something was different. In school, he was teased. He married a woman and was unhappy. They had two children. His wife divorced him when she found out he was having an affair....with a man. Now, he couldn't be happier. He is with his lifelong partner. I just think nature because he was the only one out of seven children that was a homosexual. I think that if it was nature that his brothers and sisters may have tendencies. Also, my cousins grew up with my uncle and his partner. You'd think that this environment would cause them to become homosexual. They're both straight with one happily married and the other is engaged.