Since some homosexuals like to argue that they are "born this way" and suggest that homosexuality has a genetic basis, I would like to argue that so-called homophobia also has a genetic basis, which is quite simple.
The background is the hard part....
Ovulation in human females is a vestige of the estrus cycle that early humans invariably possessed. Much like other animals, early humans could not expend energy on things like leisure sex because their energies had to be conserved for things like survival, reproduction and things which improved the inclusive fitness of their tribe.
It wasn't until later on, when humans had attained the means for leisure activities that recreational sex became (evolutionarily) feasible.
Homosexuality for whatever you think of it is just a deviant form of recreational sex (sex with no genetic offspring.)
Your sex drive is really driven by your DNA, and the desire of the DNA to replicate itself.
Hence the genetic basis for so-called homophobia:
Any homosexual union will deprive at least one set of grandparents out of having their DNA replicated via grand kids. This is not even taking into account the joy that grandchildren bring to grandparents. At least one set of grandparents in homosexual unions get cheated out of their genetic interests, period.