This is a sincere attempt to understand how homosexuality fits into the theory of evolution. Survival of the fittest doesn't explain why there would be gays and lesbians. They are not likely to reproduce especially back when we were a younger species. Please do not say it's bad DNA because that would be very insulting to the LBGT community.
There is no such thing as “bad DNA”. DNA carries no morality, DNA does not have the capacity to bother what it is doing or not. Genes that give the Lou Gehrig syndrome ([wiki]Amyotrophic lateral sclerosis[/wiki]) are as valid as any other gene.
Next, let me say that when I use the word “mutation”, I simply mean “one that differs, beneficially or otherwise.”
We are, for the most part, unaware of what genes we carry. We are even less aware of what genes our partners carry. We breed away quite happily and, quite independently, the genes get on with their job. This ignorance of our genes causes mutations to appear throughout all generations.
Let us assume that LBGT traits are genetic – it seems likely, but there is, as far as I am aware, no real evidence to this effect.
The thing is with genes that combinations of them are not expressed until the actually combine, and even then they are expressed in different ways. You can be a carrier of what is seen as a defective or beneficial gene but not pass it on through your partner. If your partner has a gene, which, in combination with yours, produces a benefit or disadvantage, the active genetic code is passed on some or all of your offspring. The offspring may then either not have the gene, have it as a carrier, or actually manifest the gene as a mutation.
By the time the affected offspring is manifesting LBGT traits, the parent has already bonded and so, nothing is done.
A couple of hundred years ago, if you were LBGT, you were either killed or live a full and happy life – either way, as the possessor of LBGT genes, you did not pass them on but your siblings, who stood a fair chance of possessing carrier genes, unknowingly continued the gene’s expression into future generations.
There is no “purpose” to LBGT genes. As I said above, genes are simply “there”.
So when you ask, “How [does] homosexuality fit into the theory of evolution?” the answer is: “It demonstrates the way genes work.” It has no “purpose.” Not everything has to have a “purpose.”Evolution is blind and unthinking. You may as well ask, “How do blue and brown eyes fit into evolution?”