I don't agree the basic argument for same sex marriage is for the government to "sanction" it, I believe the 14th amendment and equal rights has always been the argument. I've not seen anyone argue it's beneficial to family, society, nor individual much less try to show evidence(that doesn't exist) that it's somehow beneficial. Besides, I can show where a one man/one woman marriage is the MOST "beneficial to society." I believe we can provide for these extra benefits you speak of through domestic partnership laws, marriage should be reserved to the definitions society places on it.
If they were abusing their child, they'd be engaged in illegal activity, that does affect me. Furthermore, how long before they abuse someone else's children. My argument asks someone to show me how the woman living next door taking on another husband affects anyone or anyone elses's marriage, as THAT, rather than you goal post move, is the argument.And your argument that, just because what happens next door doesn't affect me means that it should not be inhibited is faulty and ridiculous. That is like saying that if your next door neighbor was abusing their child in the sanctity of their own home, without any impact on you, it would be OK for this act to be legal. See how foolish your argument is?