If homosexuality determined by genes how come there are homosexuals today even though they didn't reproduce throughout evolution?

Homosexuality is not determined by genes. The theory is that homosexuality is associated with a combination of gene sequences (not individual genes) with gestational hormones.

Gene sequences can occur and recur regardless of whether certain individual genes become rare or even extinct.

If homosexuality is genetic, born to parents that aren't, that would mean the gene is recessive. Suppose H is heterosexual and h is homosexual. If two Hh's marry and the kid gets h from both parents, s/he is then gay.