If evolution is true, how can homosexuality be genetic?

Your assumption is that homosexuality is on the rise. Homosexuality was widely practiced in most cultures before its repression by religious dogma. We also know that marriage to a woman was standard for procreation. Males both homosexual and straight reproduced. We are not necessarily seeing an increase in homosexuality but rather its social acceptance as a mating determinate. If there is an evolutionary effect then it would be generations before it would lessen in the genetic pool since it would exist as a repressed gene in straight as well as homosexual descendents of our ancestors. So homosexuals also have straight siblings that would carry the gene but it would be a repressed gene that would only show up when the straight sibling marries another straight sibling with the same repressed gene.
The other assumption that you make is that continuation of a species is best served by numeric increases alone. Humans are an apex predator and if there are too many for the food supply then a larger population is not necessarily the best thing for species survival. So an increase in homosexuality with a decrease in new births would not disprove evolution but would serve as an adaptive behavior for the species.
This is the danger when we attempt to apply logic to real world phenomenon. We must first understand our premises are guided by our paradigm. To take the idea that evolution is purely guided the survival of the physically or mentally fittest individual is a limited understanding of the theory and how species best respond to a limited or or changing environment. If I am correct then this would not be a defect. What is a defect in one environment is an asset in another and better insures that the species has a better chance of survival. .

This is a scientific question rather than a philosophical question. One can imagine an evolutionary explanation along such lines as: The community, including children and those who will later have children, is more likely to survive if there are a few people who don't procreate, because some jobs useful to the community are best done by people who aren't busy raising children. I know no reason to think there's been a sharp increase in homosexuality. I don't know whether sexual orientation is genetic. I do know that as a young person I found myself feeling sexually attracted only to persons of the opposite sex. That was something that I found happening, rather than a choice, but I don't think it follows that it must be genetic. Genes are not the only influences on a person.