You know, strictly speaking madhatter is correct. That is, an exclusively homosexual person is not going to propagate any genes, and thus their share of genes are lost. Homosexuality does not propagate itself. Rather the genetic causes of homosexuality must be advantageous to heterosexual propogation, and there is the source of propagation of the genes. In that sense, homosexuality would continue not because homosexuality is propagating itself, but because it is a side product of an advantageous gene set.
But again, that does not argue that homosexuals provide no value to society to the extent they should be denied marriage.