In debates about homosexuality, people often argue that it makes a difference whether individuals are born as homosexuals or choose to be that way. Attorney General Eric Holder, in a letter to congressional leaders explaining his stance on DOMA, stated "while sexual orientation carries no visible badge, a growing scientific consensus accepts that sexual orientation is a characteristic that is immutable." It is for that reason Holder argued that homosexuals should not be discriminated against.
Slate recently covered a study, which concluded that people who have an older brother are more likely to be homosexuals, strengthening the biological basis argument (the study found the effect was biological rather than social because it held for adoptees, too).
However, “I’m born this way” is not the reason individuals shouldn't be sent to camp to make them uninterested in those of their own sex, nor is it why you shouldn't block anyone from marrying or adopting. Who you sleep with should have nothing to do with other people. The most reasonable arguments against people acting on homosexual instincts are based on the premise that they affect not only those participating, but others in society. But given current Western sexual norms, and lots of evidence that people who have sex with people of the same gender aren’t very different from those who do otherwise, this premise is pretty weak.
The biological argument especially hurts those who don’t identify as homosexual, but who have an interest in members of their same sex. People who identify as queer shouldn’t have to be stuck with an essentialist label like heterosexual or homosexual. Scientific justifications have a fraught history. Just look at arguments that homosexuality shouldn't exist because it "isn't natural." Lots of troubling consequences over the years have come from a belief that biologically, certain sexual behaviors are natural and others aren't.
The idea that we shouldn't discourage behavior with biological bases is also problematic. That proclivities toward violence or severe learning disabilities have biological bases does not mean society should ignore them.
And if you’re willing to accept that everything we are or do is the result not of the choices autonomous individuals make but a mix of biology and circumstance, the whole question seems silly. But even if you’re not a full-on determinist, the biological justification should be troubling. Saying that behavior is fine as long as it doesn't hurt others is a far better guiding principle for a free society.