Not necessarily so.
Firstly you have no idea what (if any) genetic basis there is for homosexuality, and therefore can't arbitrarily rule out that it is present in a population.
Secondly you are assuming that a homosexual individual will never pass on genetic information, which is a pretty big assumption given that (A) we have documented observations of animals switching their sexuality back and forth under certain conditions, and (B) gay humans do it all the time.
Thirdly you are ignoring the fact that the individuals in populations that have complex interactions are liable to adopt behaviours which are not specifically related to their own biology, depending on the needs of the group.
What theme specifically?
Both.
No it isn't; it's based on a Darwinistic view of evolution only, which is nowhere near the whole picture.
We've already done this dicussion over and over in the various long threads on homosexuality.