I'm going to step in here. Let's start by saying that one of my stepkids is gay and lives with her wife, because same-sex marriage is now legal in Louisiana. I was prepared to defend my step-daughter until it turned out I no longer needed to do so. So I had a vested interest in studying this to be sure I understood it all. As noted in another thread, one of my best friends in my college days was gay. And working on Bourbon Street as a musician, I met a lot of gays.
Current research says there is no "gay" gene, not even as a recessive. But to find the current direction, you have to look back to the 1990s to a U.K. research study using Positron Emission Tomography - the PET scan that can map out a living human brain using radio-isotope injections. So... look up the following with your favorite web browser: Homosexuality + brain scan Be sure to include the "+" sign.
You would find a UK study that identified that there is such a thing as a male or female brain based on configuration differences identified by PET scan. Men and women literally have different brains and it IS a structural difference. The initial study found the differences, but a follow-up study found interesting cases that (serendipitously?) had genitalia of one gender but the brain configuration of the other gender. And when the obvious questions were asked? Yes, those cases were gay.
We must remember that when the brain says X, the body goes along with it. When you get sexually aroused, it is because your brain triggered that response. And if the brain says to go after the beautiful blonde bombshell or the handsome husky hunk, you are going to do it. As was noted in this thread, you DO have a choice - but it isn't the gay/straight choice. It is the go/no go choice.
Since the PET scan can see the structural differences and those differences relate to gender behavior, then the gay/straight dichotomy is something that has been with that person since birth. Because NO AMOUNT of choice will "grow your brain" in a different direction. Homosexuality is a condition of birth. As it happens, brain formation starts at about 6 weeks of pregnancy. And that means that by the time you popped out of the womb, your path was already directed with respect to gender behavior.
If you look up that topic, don't be surprised to see a bunch of religious types trying hard to discredit the findings. Not surprising, since the truth - if widely disseminated - would undercut yet one more issue in their holy book. And they can't have THAT, now, can they??
As to the revulsion reaction, I assure you that for gay women, seeing men and women having fun together has a certain level of repulsion as well. However, I also believe that the revulsion reaction is learned behavior, not instinctive. (The latter is my OPINION stated explicitly as such.)