I can tell you, OP, as a scientist in the relevant field AND a homosexual that what you try to push here is total nonsense and has nothing to do with science. If homosexuality was somehow just harmful to the overall fitness of a person we would expect that it would vanish from the population sooner or later due to purifying selection. This obviously didn't happen. Now there are several hypotheses why that is. Just a few I can remember: Homosexuality is not selected by individual but group selection (bit controversial, but still) which means that it is most likely a mechanism to prevent overpopulation or to ensure that at least a few members of the population have reason to help the other members or their off-spring without lowering their fitness (who are related to them and therefore share genetic characteristics with them, slightly elevating the homosexual individuals fitness to a degree). Alternatively Homosexuality is an epistatic phenomenon and/or has to do with genetic linkage in general: The alleles that effect a humans sexuality in such a way might be connected to genes that increase fitness to a high degree (for example a gene that makes heterosexual female carriers of the second gene more fertile... which has actually been discussed as a possibility). Either way, since homosexuality seems to have a relatively stable phenotypic frequency we can assume that it is at a point in which selection is "satisfied", less homo- and bisexuals might even be lowering a groups fitness/ might lower the frequency of the associated genes. By the way, bisexuals are also something to consider. Which you don't do.
Religions are often very strict about sexuality out of a multitude of reasons, none of them having to do with knowing that it's somehow morally or biologically bad: Taking control over a thing that's as personal and as unchangeable for people as their sexuality allows priest castes (who are often celibate to not fall into the same trap) to have power over them. In addition to that most homophobic religions were tribal in nature when they began and a tribe is only as good as the number of its members (since they are needed as soldiers, workers etc.). Therefore any perceived threat to the proliferation of their populace is seen as immoral and without our knowledge of evolution we have now (which as I said before show that homosexuality is beneficial one way or the other for a group) they just banned those things (like homosexuality).
Then again, you are commiting a natural fallacy either way if you want to argue against (and for too) homosexuality by pointing at nature and saying "it happens/doesn't happen there too" and then concluding some kind of law for or against it. Homosexuality, like many things, shouldn't have a moral stigma (neither should it be seen as something to aspire to, it just is, nothing more, nothing less). Since it is a characteristic people can't do anything about and which doesn't hurt the rights of others, your discrimination is unjust.
Last but not least, Germany changed its laws regarding civil marriages, not religious ones (which as a secular state Germany doesn't have control over anyway). So there is no "redefinition", at best an "expansion" of the concept, purely law-, not morality- or philosophy- or religion-based.
Please inform yourself better next time. Or at all actually.
