Assorted answers that would occur to anyone who thought open-mindedly about the problem posed for a few minutes:
Just because a particular trait makes offspring less likely it doesn't follow that it cannot persist in the population. Homosexuals still have the physical equipment to be the biological parents of children.
Homosexuality may be expressed by several genes, and may form a continuum rather than being a straight on/off switch.
Perhaps the assumptions of the question are wrong and homosexuality is not genetic at all.
Perhaps the principal genes for homosexuality are recessive. Plenty of traits survive in the population for that reason.
Latent genetic homosexuality might be triggered into actual homosexual behaviour by certain environmental events during one's lifetime, and people who didn't experience such events would grow up straight despite having one or more of the homosexuality genes.
What I find more curious, is why are so many creationists obsessed with homosexuality, to the point that again and again it is dragged into crevo debates for no discernable reason?
Yet another possiblity re homosexuality is that it's a linked trait. It may not be adapative, may even be maladaptive in itself, but closely associated with some trait that IS adaptive. Let's say (just as a merely illustrative "for instance") that the genetic factors associated with homosexuality only actually cause homosexuality 25 percent of the time; the other 75 percent of the time they cause individuals to have greater empathy with the opposite sex, and a much improved "fashion sense," both of which qualities make bearers of the "homo factor" more successful in securing mates and reproducing. Clearly such a set of factors might be transmitted and even favored by reproduction and natural selection.