No gay gene has been found, and it has been looked for.
No proof has been offered of any intrinsic physiological difference, present at birth, that distinguishes gay from straight consistently.
Experts in the field are divided in their opinions on causes.
We have Orion, who is homosexual, rightfully noting that that any attempt to say "all gay people are gay because of this one cause" is to ignore a massive amount of individual variation, and to ignore that individuals have the capacity to choose or change in at least some cases. I agree. Pointing to any one thing and saying "this is the source of the gayness" is probably ridiculous.