Well, first off, sexual preference and gender aren't the same, so that is a little off topic.
It sounds like you are reading conclusions into actions that could have multiple interpretations. Someone was converted to gay...or were they already gay and in the closet? Someone converted to straight...or did they go back into the closet in order to fit in, have a family, due to religious beliefs, etc.? Or they could be bisexual in both cases. Or maybe in one case or another it was a phase or act of rebellion. People are complicated and it would be close minded to dismiss any of those possibilities just because it confirmed by existing beliefs.
Sexual preference is generally thought to be a scale. Some people may not have a strong preference one way or the other and may be able to change. Others might not. Your sample size of whatever isn't proof in all cases. Sure, people can be brainwashed, but there is either an underlying starting point to their preference, or its all programming. If its all programming, then you aren't 'fixing' them, you are changing their programming based on your beliefs. Sounds super skeevy to me.
In the end, nobody really fully knows what is going on in someone else's head. Sexual preference and internal gender identity can't really be proven. For me, regardless of what science says, you can't prove a negative. If they say in their heart of hearts/sprit/soul they are actually a female...how do you prove or disprove that? So my choice is either to decide that my experience is the only valid one, or to respect that they might be right. That's my starting point. So even if science didn't support my beliefs, unless it can actually disprove them, it is a matter of choice and freedom and to a degree a matter of faith and belief.
So, the fact that most studies I've read conclude that there are biological and psychological reasons to believe that being Transgender is something people are born is an unnecessary bonus.