I just had a thought and from that googled the biological origins of the practice of monogamy. here is the first that popped up on google. I think they both got it wrong.

Because we know that the selection of a mate in early adulthood has to do with the not so conscious desire to even out the gene pool with bad traits that one might be carrying, the mate selected early on is the proverbial “opposites attract”.  I think that the opposites attract for that reason, and it makes it tough to finish out a 50 year marriage with someone who is definitely not your soulmate.  So how about this… because it was observed early on (those old guys knew a lot more than they are given credit for) that selection of a mate in later life, that is, when one gets past the “opposites attract” stage, and finds a mate that is more compatible in terms of temperment and drive, and in other ways, that the offspring sometimes got the worst end of the deal.  Too many “hyperactive genes”  to much “depression” not enough “energy” etc.  So the second-choice marriages produced offspring that were less able to “survive” and be “fit”.  Do you think that perhaps those observations may have forced the coupling of early mates, to insure a better gene distribution.   Ha ha… just thinking here.  No way to prove (no desire to prove either) just thinking out loud.