The problem is that while it is obvious to you that sexual orientation is a matter of birth and not choice, it isn’t to, to be honest, the vast majority of people on this planet.
And also, just to put things in perspective, even the science isn’t fully convinced. Most evidence tells us it’s something from birth, and my personal life anecdote tells me I’m bisexual since the day I was born, but truthfully we don’t have any hard evidence to prove it, since it is nearly impossible to prove.