Science behind being gay?

Mary • My beliefs and opinions are constantly evolving
Today in my AP Psychology class we learned about sexual orientations such as homosexuals and apparently there is a theory to have believed that there is someone science behind what makes a person gay. 
Now it did not say that it was wrong, I'm pretty sure the video made it clear that being gay was not a choice and that people are born with it, and nothing's wrong with that.
It was said that even though it's more of a stereotype, it is common for gay men to "move a different way". Like how we sit, talk and walk. The video also stated that you could determine the sexual orientation half or most of the time with how the child's behavior is like at such a young age, and that the more older brothers or sisters you have, the more likely you are to become a homosexual by 1%. 
Another reason to believe why people were homosexuals is bc it was due to genes and hormones. 
Now I'm pretty sure these are only theories all based on experiments that they did, and they're not trying to say that being gay is a mental problem, they're not saying it's wrong, they're not saying it's a disease, but they want to know and find out what makes someone a homosexual. 
Do you believe that there is some type of science behind being one? Or do you have other thoughts on why someone is?
I ask this bc some of my friends were complaining about the video saying that the video was wrong and that its shaming people for being gay