Since a body's sex can be altered and love does not restrict itself, any two consenting adults may marry.
Does this include close relatives like mothers and sons, sisters and brothers? And why only two consenting adults? What is a secular argument against multiple spouses?
As I mentioned, I suspected there were things I was missing considering the lateness of the time.
Due to potential for disastrous birth defects, blood relatives would be prohibited from marrying. However, after the existing marriage contract had expired, unrelated stepparent & stepchild (both 18+) could then marry.
As for multiple partners: it would be a very difficult sell in the USA. I wouldn't have a problem with it as long as the ratios were fairly balanced but I don't know how you could legislate that. By balanced I mean no situation where it revolves around one person with all married just to him (or her). (Can you say beginings of a psycho cult?)
There are other logistics too with multiple partners. Does everyone marry everyone else? Do only those who anticipate having sex marry each other?
I think American society would be better off if we managed to lose the idea of the (fictional) nuclear family and went back to having something more like an extended family. From history I know that people used to live where they were born and they were surrounded by family. While that didn't make things perfect, I think it helped a lot. And, as someone mentioned, extended families are developing again anyways - why fight that? But the theists argue that a family is one man & one woman with their kids and work to destroy anything else no matter how beneficial.