I hesitate to step in here, but I find the naivete of the question breathtaking. As some of the other posters have suggested, things are a lot more complicated than the headline "Sexuality: Nature or Nurture?" acknowledges.
For one thing, there are at least three aspects of human sexuality that are in complex relation with each other: biological gender, psycho-social gender identification, and affective orientation. Biological gender, although it seems simple enough (male or female), is a mix of genetics (X/Y) and fetal development processes, which can result in one gender according to genetics and another according to gonads, or a mix of the two genders. Psycho-social gender identification is the question of whether an individual considers themselves male, female, or a combination of the two. The relationship of biological gender to psycho-social gender identification is not at all clear; we see examples of a contradiction between the two in transexual individuals and some (but not all) transvestite individuals, some of whom declare that they always knew they were the wrong gender, suggesting some kind of biological basis. Some of the famous experiments on "feminizing" male rats with heavy hormone treatments may provide a clue to this contradiction--except that transexual/transvestite individuals don't typically show such hormonal crossing. On the contrary, pre-op and post-op transexuals have to maintain a steady diet of hormone supplements to maintain their chosen gender identity.
Affective orientation is the most complicated. In spite of the rhetoric of LGB, and the insistence by many people that they always liked girls, or they always liked boys, it's clear that, for many people, affection (up to and including sexual attraction) is much fuzzier in its targeting than that. Some people feel strongly one way at one point in their lives, and then swing in another direction later; other people are more settled in one orientation but feel attractions in another direction that are never strong enough to be acted upon, but are there nevertheless. Lay affective orientation on top of biological gender and psycho-social gender identification, and you get human sexuality in all its variety.
Biologically we as a species may not be as flexible as clownfish, but mentally and emotionally we put them to shame.