That is where you and I disagree, bonding between two (or even multiple ) people in general is beneficial to society even if they can't produce children.
I am not sure that an empirical case for net societal benefit can be made but it would be fun to see if one has been produced.
Production of children can be done now without the actual need for a male/female relationship. Why limit marriage to that?
You mean like factory production of kids? While we may have the technology to do so, is a human being produced like that really human? They would not have the early imprinting of being carried in a womb, nor the other sensory inputs that a traditional baby would have.
Learning starts at the earliest age. What do they learn and what is the result in terms of being human rather than just a collection of cells making up an organism.
Single sex couples that have responsibility for parenting and, to much the same extent, single parents have similar issues to address.
Having two sex households and a loving relationship between the parents goes a long way toward avoiding major issues. Does adding one or more partners (and they don't have to be equal, no human relationship is ever equal) enhance or reduce the possibility that the child will be happy and well adjusted for their lifetime?
I think that biological bonding between birth parent and child is of a different character than that between a child that is adopted and a foster parent. The fostering can be wonderful and I admire those who take on such a responsibility, but I also know that the odds for failure at many levels are much higher .
Heinlein had some strange views on government and society. It certainly made for good reading, but I'm not sure how well his ideas would work in practice.
He was a real libertarian and a visionary. More people should read his stuff.