"But ignoring that, the big aberration is that, by logic, homosexuality would wipe itself out if it were purely down to genetics..."
Genetics are not as simple as you make out. Have you ever heard of recessive and dominant genes? The sort of genes that skip generations? The ones that propogate through women while only effecting men?
Our understanding of genetic combinations is not complete enough for the above statement to be complete and valid. Actually, even with our limited understanding the above is not necessarily correct.