Getting back to the group marriage thing, I believe that in "The Moon Is A Harsh Mistress", the marriages were polyandrous (usually.)
Heinlein posits that the Moon was set up as a penal colony. It's pretty standard that there are more men than women in prisons. So he makes a throwaway comment about the most common marriage on the Moon being "a troika" - one woman and two men.
So, for the sake of argument, maybe Clark's group marriage has more males than females. Then Lois gets involved, and Clark finds himself becoming very very possessive...
