This is the stupidest argument for gay marriage, and it doesn't surprise me it comes from Breyer, since Souter is no longer there to be the village idiot.
Why don't we allow brothers and sisters to marry, or close relations? Precisely because of the harm if they should have children. But, according to Breyer, many marriages don't have children, so clearly there is no rational basis to deny brothers and sisters the "right" to marry.
And since the concept of traditional marriage is that you get married, and THEN have children,and not the other way around, under the "Breyer" standard marriage makes no sense, because NOBODY WHO GETS MARRIED has children at the time they get married.
In reality, the point of marriage WAS to have children. There were few people getting married simply to guarantee themselves a partner. That some people can't have kids is of no consequence -- most can. That some people choose not to is of no consequence -- most do. And that marriage doesn't always work out the way government had intended, doesn't mean marriage has to be modified in a way that guarantees that it won't work the way government intends.
Since there is no way to guarantee that a particular couple WILL HAVE CHILDREN before you grant the license, in the history of the world up until now, marriage has been defined precisely in the way that was most likely to lead to the outcome. So, we don't let brothers and sisters marry. We don't let same-sex couples marry.
Frankly, there is no particular reason for the state to recognize the marriage of two people who are past childbearing age. But marriage has always been allowed between unrelated men and women.
Even in our darker history, the rules are based on children -- we didn't allow mixed-race marriages NOT because we were concerned about a white guy having sex with a black woman, but because of irrational fear of mixed-race children.
The bad part of the rest of this summary of the oral arguments is that most of them have nothing to do with what the court is SUPPOSED to be deciding -- NOT whether it makes sense to ban gay marriage or allow it, but whether the constitution REQUIRES that two people of the same sex be allowed to get government sanction for "marriage".
There was a particulary “funny” exchange between Scalia and Kagan on this subject.