Just past allowing consenting adults being allowed to marry each other, and just short of allowing adults to marry children, animals, and inanimate objects.
Its easy to say that, but more difficult to give a reason why it should stop there if it doesn't stop at homosexuals and group marriages. People say, "LET THE HOMOSEXUALS HAVE WHAT THEY WANT AND BE HAPPY." Well, why not let people who are in to bestiality have what they want and be happy?
As for consenting adults, a man could marry 50 consenting women. Is there anything wrong with that?
Why shouldn't "adults" be able to marry mature "adolescents"? The only reason most people can think of is because the law says that adults can't fuck children (and they can't enter legally binding contracts...). Well, WHAT IF THEY DONT FUCK THEM until they are 18? Are all marriages about sex?
And the law isn't really a viable means to assess competency and the ability to consent, so maybe some adolescents should have the right to be able to marry