Capitalism has two factions:
The first faction (that has existed for longer) wants men to buy women as property.
The second faction (that is more recent) wants men to rent women as commodity from a capitalist service provider.
OP strikes me to have aligned with the second faction. And might be trying to manufacture consent by rebranding capitalist development as socialists. This is mildly irritating. Saying this is about monogamy vs polygamy is missing the point, because it lacks class analysis.
Socialist will not pick one of these capitalist factions, they are both very reactionary forces. Commodification of social relations isn't any better. The propertarian faction wants to reduce women to a household appliance and a baby maker for the owner. The commoditarian faction wants to reduce women to a whoring service and surrogate womb provider for the customers. There is no progress here, it's just variation in the method of subjugation. Both methods of subjugation are instruments for the reproduction of class society, and a way for the ruling class to control access to sex and reproduction.
Socialism is going to put the interests of the working class above other interests. If i had to make a guess, a lot of workers, both man and women, will continue to form something similar to traditional families, because that's what they prefer. Other workers will chose to live in different social arrangements. We will try to build appropriate structures regardless. Socialism will have fewer constraining factors because it only needs to reproduce it self at the level of society. The additional hurdles that have to be put in place for reproducing a ruling class will fall away. Without the extra burden of a ruling class social relations will improve somewhat.