Chandler Burr is probably the best advocate for the notion that homosexuality is a biological, structural trait that is clearly not a choice.
The above link provides lots of interesting information. It gives another extremely useful analogy to homosexuality: left-handedness. The analogy however, shouldn't be overused, given that left-handedness in no way implies sexual behavior and homosexuality does. The analogy is most useful in demonstrating the unchosen and "built-in" (structural) aspect of homosexuality. In other words, saying something along the lines of (as the religious right commonly does) "every time a homosexual chooses to have sex, he is making voluntary, conscious choice, thus homosexuality is a chosen phenomenon" may be technically true but makes about as much sense as "every time a left-handed baseball player chooses to bat left-handed instead of right-handed, he is making a voluntary, conscious choice, thus left-handedness is a chosen phenomenon."
As the Burr article demonstrates, the parallels between the two orientations are striking. If homosexuality is a "behavioral" phenomenon, then so too is left-handedness. In terms of identical twins, left-handedness, like homosexuality has high concordance rates, and unlike with eye-color or hair-color, identical twins often have different types of handedness. In fact, as Burr notes:
But -- surprise -- with sexual orientation, both twins share the trait homosexuality more often than they do left-handedness -- yet no one would claim this is evidence that left-handedness is a "chosen alternative lifestyle" because left-handedness isn't seen as a moral issue -- any more.
We've often been told that "there is no gay gene" because a "smoking gun" gene has yet to be identified. But guess what, no "left-handed gene" has been identified either. And there is no more evidence of a "left-handed gene" than a gay gene.
Here's another thing most people don't understand: We don't need to discover the genes to know that you don't "choose" your sexual orientation any more than we need to find eye-color genes to know you don't "choose" your eye color. We're closing in on the genes that make us heterosexual or homosexual. Geneticists, using the clinician's research, have begun to look for the underlying biological determinants of heterosexuality, bisexuality, and homosexuality. In ten, twenty, or thirty years, we'll probably have figured it out. We've got the basics already. In early 2005 in the highly-respected biomedical journal Human Genetics, the team of Dr. Brian Mustanski of the University of Illinois at Chicago identified three chromosomal regions linked to sexual orientation in men: 7q36, 8p12, and 10q26....No one questions that blue eyes occur more frequently in Caucasians than in Asians, but we don't know this by finding the genes for eye color; we know it by clinical observation of the distribution of eye color in people all over the world. No one questions that about 7.8% of all human beings are left-handed, but we don't get that information from genes -- in fact, as of yet, we have no idea where the genes for handedness are -- we get it, again, from clinical observation. We don't need to find the genes for sexual orientation to know that people don't "choose" to be heterosexual any more than we need to find genes for handedness to know that people don't "choose" to be right-handed. Among scientists, this is as obvious as the sky being blue.
A couple of comments. I think that the "choice" matter of homosexuality can be overplayed by both sides. Homosexuality isn't the only other thing, like left-handedness, that can be reduced to "this isn't a choice but part of my make-up." See this article by Virginia Postrel.
For instance, if we discovered that pedophilia or the desire of men to sexually dominate women had similar biological or evolutionary groundings in human nature, that wouldn't make them any less wrong.
Ultimately, the same thing that justifies homosexuality is the same thing that justifies left-handedness. And that is this: Clearly there is nothing "wrong" with the underlying behavior. Rather, a southpaw can use his left-hand to do good things (like petting his dog with his left hand) or bad things (like beating his children with his left hand) or indifferent things (like playing guitar left-handed). Similarly, one can have moral, immoral, or morally neutral homosexual sex.
And given that there is nothing wrong with the underlying behavior, we might expect to see people who don't have the primary orientation participating in the behaviors nonetheless, if they so desire. Here is where the analogy may begin fall apart a little because just about everyone is ambidextrous to *some* extent (for instance, I -- a righty -- am typing this post with both hands), but it's doubtful that everyone is bisexual to *some* extent. I believe that a lot of folks probably do have pure homosexual or heterosexual orientations.
But, on the other hand, I don't believe that the 97% of society that self-identifies itself as "heterosexual" has a "pure" heterosexual orientation. Cross-cultural studies indicate that a shockingly high rate of any given society's heterosexual population has the ability to enjoy homosexual acts to *some* extent (Maybe 1/3?). But again, this shouldn't surprise. An equally high number of right-handed folks probably have the ability to "learn" how to function lefty (like the right-handed batter who switch hits) with *some* degree of skill. But we also understand that the underlying right-handed orientation doesn't change there. And we further understand "real" ambidextrous folks to be those who are equally skilled and comfortable, right-handed as left-handed, not those folks who are primarily right-handed but have the ability to function as lefties to *some* extent. And I've argued on this blog that bisexuals should likewise be understood in this way.
I think the "choice" issue is also relevant in this respect: Given that homosexuality, like left-handedness is unchosen, and given that there is nothing wrong with either the underlying left-handed or homosexual behaviors per se, society then therefore has a duty to accommodate homosexuals as we do southpaws.
Imagine a baseball game where the "rule" was everyone had to play right-handed. Let's say a few righties liked to switch-hit (I'm not much of a sports fan; they do this to confuse the pitcher?) and they'd have to give that up; not much of a sacrifice on their part, right? Similarly if society says, "heterosexual behavior only," then those heterosexuals who might like to engage in homosexual acts for release might have to make a small sacrifice. Pure heteros would be giving nothing up. But the "righty only" rule obviously dramatically and unfairly impacts the left-handed player. So too does a "hetero only" rule unfairly burden homosexuals.