There is not one single scientific proof that gay people are ‘born that way’. However, it is a widespread assumption constantly proclaimed by the media as the ‘truth’. I did try to find and read some info about this, and there has been some research.
I read one research that was trying to prove that gay people have a different brain. However, they used both healthy and sick gay men (who died from natural causes or from AIDS), and they could not prove if the changes in the brain were due to being gay or were caused by AIDS.
There have been other attempts, e.g. one research proved that people who tend to behave like members of the opposite sex as children (e.g., girls who behave like boys, or boys who behave like girls), are more prone to becoming gay as adults. However, nobody tried to find the reasons, WHY did they behave like this when they were children - family problems, how they were raised, if they suffered any physical, psychological or sexual trauma, etc.
Personally for me, ‘researches’ like these are tainted, as they are deliberately trying to find something that has been there all the time, and NEVER try to see if there has been something acquired during one’s life experiences. There is a lot of research being done in this area, the modern idea is to find some ‘scientific’ reason to hide behind, and NOBODY has been able to prove that gay people are ‘born that way’.
I personally know three people who are gay, and I have some personal experience with all three of them, growing together with two of them. In all three cases, those were the younger of two children in atheist families, where having more that two children was considered wrong. So, instead of having a lot of children, the mothers decided to do and give everything to their boys, knowing they would be their last kids. All three of them were spoiled rotten - ALWAYS given everything they always wanted, never denied a thing, nobody taught them to think about others. They all learned that they can do anything they want, and there would be NO consequences. E.g., in one case, the boy would mistreat others, call them ‘mule’, etc., and then his mother would always protect him, telling the people complaining that they are wrong, arguing with them, even shouting that her ‘perfect little son’ would never do something like that, etc.
I know my experience is VERY limited, however, it does show that not raising your child right does have consequences. As far as I know, however, nobody is interested in making a research in that area, to see if there is a connection…