Alright fair enough, but then again, a child in this country is not recognized by the law as being able to enter into a legal contract until they're [insert state's age here]. It is assumed (and yeah, I'm sure some kids are exceptions), that they cannot possibly understand the ramifications of the contract they're entering into.
An ape, though it could probably sign an X and use sign language, wouldn't, in my mind, be capable of truly understanding what it was doing, thus the contract would be invalid. I understand what you're saying about the fringe groups though, and, who knows, maybe there is an ape out there who could - I'm doubting it.
Different states have different ages to enter a marriage contract, my cousin, for example, got pregnant and ran off with the loser to FL to get married at 15
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I could see how one might say "well then, isn't this seperate/not equal (marriage ages)", but I'd argue that regardless of where you live in the country, if you're heterosexual you're going to be allowed to get married eventually - gotta leave state's some say of there own, anyway.
In the end though, although I hear what you are saying about opening the flood gates, you have to consider, as others have pointed out, that blacks and whites couldn't marry each other at one point either. When they were suddenly allowed to, all the groups you mentioned didn't swarm the gates (successfully), and I don't see that happening now.
It's something to think about, sure, but I just don't see it as happening.