Nah, I gotcha. I recognize that retellings of a timeless myth are meant for different kinds of readers. Let’s just say I am not really in MZB’s target audience for a variety of reasons.
Interestingly, Henry did have a brother Arthur, so there is some symmetry there. Henry VIII did have a bastard who died inconveniently young. He was just more of a commitment type and usually married his mistresses.
So the real problem here is not monogamy but primogeniture. 
The father must have been healthy enough to live to a breeding age and was able to find a woman to have sex with, so he passed the minimal qualifications. And then his sperm won the race which was the real genetic contest.
Think of it like the NFL. The first step is the regular season. It doesn’t matter if you go undefeated or slide in as a wildcard with an 8-8 record; if you make it to the playoffs, everything starts over again. This is the equivalent of getting to the sex; you might be a super-genius with a perfect body who writes poetry in his spare time or you might be some schlub who got lucky because he was in the right bar at closing time.
But either way, your sperm is now in a vagina. It’s playoff season; the regular season no longer matters. It’s all about which sperm is best at reaching the egg and fertilizing it. The sperm that have the best fertilizing genes will presumedly win and pass on those genes to the next generation. If the father had mediocre genes otherwise, they were still good enough to get the fertilizing genes to the point where they could show their superiority.
Yeah, but there’s a certain amount of randomness in the system. And to extend the sports analogy, it’s like you’re playing football in the regular season and basketball in the playoffs: success at one does not predict success at the other. If a guy has genes that give him weak muscles, low intelligence, bad digestion, and really excellent sperm, he has a lower chance of getting to the playoffs at all, but if he does get there he has a higher chance of winning. If you have two attributes that don’t correlate, selecting too strongly for one can make it harder to select for the other.
But the fact that his sperm is in the vicinity of her egg is proof that all of his other genes are “good enough”.
There are genetic advantages to having all those other genes. A guy that looks like Jon Lovitz might have great fertilizing genes but only get rare opportunities to use them. Meanwhile, Ben Affleck may have weak fertilizing genes but they have a lot more opportunities to succeed.
But is he also proposing to give me ‘hurpies’? And what does it have to do with my/his mother?
I thought it might have been drug-related, but yours is a viable guess too, I suppose.