Why stop there?

I support most of President Obama's stand on legal gay marriage. I do think, even if he believes it is a state issue, he could come out for a change in federal marriage rights for gay couples. Otherwise, I agree with him.
The right makes a lot of analogical arguments on the subject; if you let gays get married, why can't you marry a duck, or a child, or five people? But, as I see it, these arguments don't stand together.
Part of the reason gay marriage has gained such acceptance is that the opposition has not given a good rationale against it. They tried marriage will be destroyed, but people didn't buy that. They tried it will mess up the children, but research was done and nothing backed that up either. All their arguments evaporated when looked at seriously.
But now I hear gay proponents making similar statements. Of course, gay marriage won't lead to child marriage, or inter-species marriage. There is a sharp dividing line there - consenting adults. Rights for consenting adults have NEVER led to the same rights for children or animals. Our legal code is chock full of age limits for rights. It is a deeply accepted part of our legal code. And obviously animals are not able to consent to a marriage type relationship. These are CLEARLY different situations.
However, rights of different numbers of consenting adults is a different matter. If we legalize gay marriage, why not multiple marriage? Here I have heard gay leaders say "because marriage is for two people" just as the right said "marriage is for a man and a woman". Why??????????
Why does one statement stand up when the other doesn't? I have yet to hear reasoning. They might fall back on the children as the right did, and I am not sure if there is research on children of multiple partner households and what it showed if there is. But since it has been practiced in many places in the world over many centuries, it must have something to say for it.
I think we must face the fact that legal gay marriage MAY lead to legal multiple partner marriage and examine whether that is such a bad thing. In my book it's not.