Remember that for a trait to be passed on, it doesn't have to be beneficial. It can be "not un-beneficial." That is, if it doesn't stop them from spreading their genes, the trait is carried on.

If a homosexual man is married and has kids but decides when he's 40 that he's been homosexual his entire life, it's an evolutionary moot point. He's already had kids and passed on his DNA. And since homosexuality hasn't been proven to be exclusively genetically based, there's nothing to suggest that his kids will or won't pass on those traits to their kids (the original homosexual's grandkids).

Another example even zanier: Say I'm born with a long and fully functional tail. The doctors, horrified, cut if off so I will be a "normal" kid. I grow up and have kids. The tail gets passed on and from then on, my descendants all are born with tails. The tail was not beneficial, but it still gets passed on.

On the flip side, say the doctors don't cut off the tail. I get laughed at and rejected in high school. I never meet someone who will love me despite my tail and I die. Even though my tail was fully functioning and helped me paint houses faster than my competition (making me a financial success) I never meet a girl and have kids, the tail doesn't get passed on. The tail was beneficial to me, but it ceases to exist once I die.