I disagree with your views, EvangelionZeta.
Every law we have ever dealt with is about humans. You are basically arguing:
"Why should marriage be restricted to humans only?"
"Why should possession of objects be restricted to humans only?"
"Why should a driver's licence be restricted to humans only?"
"Why should the jury be restricted to humans only?"
And so on, so forth.
I will give you a reason why all of these laws are restricted to humans only and why even considering animals in this situation would be perverse:
On the grand scheme of things, adults have the most rights, children have the second most rights and animals have the third most rights. Animals are inferior to human children, so unless you're suggesting that we legalise everything for children and make it legal to marry children, then I don't see how animal marriage can even be considered. Homosexual ADULTS are more superior, more knowledgeable and more mature than animals/children/pieces of bacteria etc.
And as ninatron pointed out, laws come bit by bit. You can't just say that just because it should be illegal for the most extreme example to happen, then it should be illegal for the least extreme. You seem like the sort of person who would talk about the dangers of heroin in a debate against the legalisation of marijuana. It's just not relevant.
edit: Okay, maybe not every law is about humans, just like not every law concerns adults only (children still have rights too). But you can't expect an animal to have the same amount of rights as a human adult. Animals are inferior to humans. Gay people are equal with straight people. There is a difference.