Since certain twins (one part of the pair) can be acertained to be homosexuals even from early years, it seems to be quite a difficult matter.

At that age children do not have any incentive to make a choice (as if you can chose who you are attracted to ), yet there is little you can say about it. Upbringing doesn't seem to affect that since twins are treated equally to a great extent. Hormones in the womb should be equal as well, but we can't know for sure though. Genes are the same.

So we end up with a situation where we simply can't say that it is one thing or the other.