It's not a Church's Opinion of funcions, it is human anatomy, nature.... If Adam was Gay and Eve was a lesbian you wouldn't be here right now even talking about this, the human race would have died off long ago.
And if both were astronomers and not interested in tilling the land or raising animals, they wouldn't have had food to survive either.
There are plenty of things that work in a medium-to-large population (or even a small one) that don't work when you only have two people. Why should we conclude that anything that Adam and Eve were not able to do is immoral or unnatural?
I'm an engineer. If Adam or Eve had been engineers, they would have been useless members of their two-person society. Is engineering wrong?
In a two-person society, you need no politicians, no justice system, no police, no military, no religious heirarchy... in fact, anyone trying to engage in any of these activities would be taking away badly-needed labour from basic survival. Are all those things "unnatural" just because a theoretical lone pair of humans would not have been able to reasonably engage in those pursuits?
If it's okay for a person today to be a pastor or a priest, for example, even though it would have been untenable for Adam to be one, why should we conclude that it's not okay for a person to be gay, even though it wouldn't have worked in a situation of one man and one woman?
You claim that if Adam and Eve were homosexual, they would have died without bearing children, and therefore homosexuality is unnatural. While I don't accept a literal interpretation of Genesis, just to follow your logic for a moment, if Adam and Eve were anything other than heterosexual farmers (or potentially heterosexual hunter-gatherers) the same thing would have happened: they would have died without children. Unless you're prepared to declare every profession other than farming, hunting and wild berry-picking (and not only that, but farming, hunting and berry-picking in the manner that Adam and Eve would have done) to be "unnatural", you're committing the fallacy of special pleading and your argument is therefore invalid on its face.