asynchronous harem instinct

There is no doubt that we are a polygynous species.

There is considerably more variation when it comes to mitochondrial DNA, which is inherited only from mothers, than in Y chromosome DNA, bestowed upon subsequent generations exclusively by fathers. In other words, over the evolutionary history of Homo sapiens, a relatively small number of men produced children with a relatively large number of women. As a species, we have had a greater variety of mothers than of fathers. 

David P Barash - Is God a Silverback?

Of course it is extremely barbarian to voice this fact in our modern society. We are supposed to get married to a single person and stay married to that person until death does us apart.

Despite being suppressed, the harem instinct is nevertheless alive with us. But it manifests itself in an asynchronous form via a proliferation of step brothers and sisters from successive marriages.