I’m not sure what leads you to believe that society is in a stable position, for one. For two, whatever you’re calling “stable” has been accomplished through many things, marriage included, though sometimes not; this is not to say that a “stable” society will fall to pieces if alternatives are accepted.
Let me see if I can follow your reasoning here. You’re suggesting the only reason population hasn’t dropped yet is because people are not legally allowed to be married in some states? And once legislation is passed, everybody that was once reproducing will cast off the shackles of their dreary heterosexual relationships for relationships in which reproduction isn’t feasible (if that’s possible)? This is poor reasoning on any grounds, but especially when considering the demographic characteristics of the homosexual population (they’re a huge minority).
Just skimming my notes from anatomy class, this seems not to be true. However, same sex couples can’t reproduce through intercourse with each other. There is surrogacy and adoption.
This raises the possibility of unions that involve 10 000 or more people too, but it’s not likely. And I’m sure you’re well aware, legislation isn’t what’s keeping those who desire triple unions from sharing domestic space.
The increase in animal rights would properly lessen the chance of inter-species unions. The reason we don’t allow unions between people and animals is because there can be no consent given on such terms. You’re thinking of it like this: “If they allow one depravity, why not another.” The problem is, secular society doesn’t determine what’s depraved based on whether someone can reproduce with their partner.
You’re constructing an argument in which same-sex marriage leads to catastrophe (population decline, animal abuse, unstable societal relations). But honestly, if same-sex marriage legislation became the norm, and nothing else changed, you still wouldn’t support it because the real reason you’re against it is for your religion’s sake, so why not just say that?