I have read the classic sociological and biological studies, and met enough gays and lesbians, to be convinced that there is some kind of physical basis for a homosexual orientation in some people.
Kinsey came up with the famous estimate that about 10% of the population is exclusively homosexual. More recent studies, especially that by Laumann and the Chicago team, give a more accurate picture: 3% of the population (2 – 4% of men, and 1 – 3% of women) are exclusively homosexual. 3% of the U.S. population is about 9 million people. That is a small percentage, but a sizable number of individuals. They are concentrated in cities, where Kinsey did his work, which is why he inadvertently over-estimated the homosexual percentage in the total population.
There have been tantalizing studies that have found biological correlates of homosexual orientation, but no one knows why some people have it. There also seem to be more biological correlations for gay men than for lesbians. For example, boys born at the end of a sequence of brothers are much more likely to be homosexual; the same is not true for girls in a sequence of sisters. I am not a biologist, I just read their studies. It seems to me that there is probably not a "gay gene." There may well be something important in the sequence of prenatal hormone washes that babies get in the womb which has a great deal to do with how our brains are oriented. In the vast majority of people, brain orientation matches the rest of our anatomy. In a small percentage of people, though, the correlation comes out differently.
I am also persuaded by the homosexual friends, colleagues, neighbors, and acquaintances that I have had over the years that some people are "born that way." From the coming-out narratives that I know, this seems to be more true of men than women. In general, men tend to be more polarized in all kinds of characteristics, whereas women are more likely to be distributed across a spectrum. Women are more likely to switch orientations than men; there does not seem to be a male equivalent of the "hasbian."
More mysteriously, there does seem to be a correlation between homosexual orientation and certain character traits and skills. Here we are way out on a scientific limb. In my experience, lesbians tend to be tougher-minded and more competitive than other women. Gay men, famously, tend to be more musically and aesthetically talented than the average man. The queer eye is different than the straight guy's eye. I have no idea why this would be so, and maybe I have been misled by stereotypes.
However, and this is a very big however, biological orientation does not settle the moral question. Just because you are born with an inclination to do something does not mean that you should. As a biologist once pointed out to me, if you are born with the breast cancer gene, that does not mean you should start smoking to make sure it expresses itself. We all have all kinds of orientations and inclinations. Some are good and some are not. Our individual morality, and our social ethics, are concerned with which of our inclinations we should develop habits to promote, and which we should develop habits to control. To take an important example, people as a whole, and nearly all individuals, have strong sexual desires. If we acted on every sexual desire we ever had, the result would be bad for us individually and bad for society. We create all kinds of social institutions to channel our strong sexual desires into socially useful practices, because they can so easily become destructive.
Most people have a natural heterosexual orientation. Some people have a natural homosexual orientation. Neither group should simply act on their orientation. How we should regulate our sexual orientation is one of the most important things that we have social institutions for.