Okay, so the answer is because the scientists say so and because gay people are genetically predispositioned to be homosexual. Hmm...
Perhaps I'm being a stuffy literalist when it comes to Evolutionary biology, but why would natural selection preserve (select) the genes (of homosexuality) which did not serve the evolutionary purpose of reproduction? Why would Nature engender a sexuality that by its very condition is not conducive to the contiuity of genetic material (since gays cannot reproduce with each other biologically)?
I'll have to ask a proper biologist about that one when I get the chance...
I'm no proper biologist, but you can look to bees for an extreme example of how that strategy can be effective: the queen is the only one that breeds, and all of her sisters work to help and protect the queen and her offspring (i.e. the workers' nieces and nephews).
There are more ways to get your own DNA on to the next generation than having offspring yourself.
Does everything that is a product of evolution have an innate important purpose? Does evolution every leave anything that isn't absolutely necessary for survival behind?
No.
There are lots of mutations; some positive, some neutral, some negative. Positive traits are selected for, negative ones are selected against, neutral ones aren't selected for or against at all.
However, things aren't usually so simple as having a single gene for a single trait, and many traits have positive and negative characteristics: for example, a deer with large antlers will have an advantage during mating season (in helping him fight off rival males), but be at a disadvantage the rest of the time (since he has a pair of big, heavy, awkward things attached to his head that are a diversion of biological resources and energy to actually grow). In these cases, whether something's a
net benefit depends on the specific circumstances.