It wasn't that long ago that monogamy was the default because no one wants to have a baby from a night of netflix and chill.
IMO you have the direction of causation backwards. Monogamy is not some child rearing optimization strategy. It was a social construct that evolved because causual sex at one point was incredibly expensive and now it is not because of birth control.
Also, if I give myself to my wife as a whole (i.e. I take care of her, the home and the children) I do not have time really to have another affairs. The rest of the time I'm left with I either sacrifice to be with her or have my own time like play games or compose music. There are lots of things to be done really, and I couldn't imagine sacrificing my family and duties to pursue sexual satisfaction with other people outside of my family.
That's a good point, and true of most people I expect. The wealthy have time, though. Wealth buys time. Kings, the epitome of being wealthy historically, have always had time for affairs, and are oft remembered for exactly that(!), but a growing proportion of "common folk" are becoming increasingly wealthy themselves, freeing up the necessary time among more and more people. However, wealth inequality is high, so the opportunity of time is not evenly distributed.
Can you go into this a little more? Is there evidence (either way) that stable 2-parent households are or aren't better for kids, or that an alternative is better?