It took the American Psychological Association a few years to catch up to the state of the science on sexual orientation — nature or nurture, but here is the opinion the APA holds now,
There is no consensus among scientists about the exact reasons that an individual develops a heterosexual, bisexual, gay, or lesbian orientation. Although much research has examined the possible genetic, hormonal, developmental, social, and cultural influences on sexual orientation, no findings have emerged that permit scientists to conclude that sexual orientation is determined by any particular factor or factors. Many think that nature and nurture both play complex roles; most people experience little or no sense of choice about their sexual orientation. (Source: APA —Sexuality — Answers to Your Questions For a Better Understanding of Sexual Orientation & Homosexuality)
That removes all doubt, does it not? It is a totally safe opinion that cannot be proved wrong. Regardless of what sexual orientation one chose or not, it was either nature or nurture that imbued the individual with it.
The only question that now remains is which sexual orientation is the safest and the healthiest. Choose wisely. (More)