I think everyone has the homo gene in them and some people just have it "Activated" more than others for whatever reason. I am basing my theory on some animals that are born different and seem to attracted to the same sex within their species and they were not taught to be like that.
Comments: The only person to really answer this question is the person that is gay. As a homosexual I can safely tell you that I grew up in a home with both a mom and dad. They did a great job of raising me. I always, from the earliest I could remember liked being around boys more than girls and felt attracted in that way to men more than women when I got older. It was just me, I was born like this. No one taught me or forced me to be gay.
A look into the genetics of a trait tells us that there are such things as “genes for sexual orientation”.
Except for the religious zealots being gay is accepted as part of life so how it came about is not as important as being accepted. Personally, I have no view on it but one thing is for sure, no one makes up being gay, its real.
All human behaviors are complicated and exhibit genetic and environmental influences. It is clear that homosexuality has a strong genetic basis. Most people who are gay know it when they are young and have the same life experiences as straight people (Except for fear of coming out and discrimination for being gay). As for the complex sexuality between gay and straight that is now more out in the open, I believe it arises due to a mix of the genetics of sexual preference, other genes related to emotions and behavior, and life experiences. And one or the other may be stronger in individual people. Honestly our culture dwells on this way too much, as it does with all forms of discrimination, to the detriment of productivity. Hopefully one day we'll stop obsessing about insignificant differences among people (I.e., traits that have nothing to do with people's contribution to society).
My belief is that I don't personally think anyone is born gay. I think it's just a prefrence for people when they start experiencing life. Everyone has different views and likes.