That is not the argument. You specifically said, "Homosexual activity in animals is never monogamous as the breeding and care of offspring still depend on both sexes playing a role in the care of offspring.
The only animals that exhibit monogamy are exclusively male-female for the production and care of offspring.
the statement that homosexuality is natural because animals do it does not apply to monogamous relationships, which is inherently what marriage is about."
In which case you were proven wrong by two different postings
493 and
498 specifically in which it was shown that male penguins will and do participate in a monogamous lifelong homosexual relationship in which, given the chance, will undoubtedly raise offspring as if they were a "normal" mating pair.
Also I believe your argument has been about survival of the species and not a particular genetic line. In this case homosexual couples have and do adopt children and raise them to be healthy productive people thus they do contribute to the survival of our species while in monogamous relationships.