I mean, if I was in the King place I wouldn't want twice as many mouths to feed. You wouldn't know how or even if you get saved in those sort of scenarios, so it is better to not rise population if you can't reasonably sustain it. That's why not making girls pregnant en masse is rather smart thing to do. Don't know if the guy thought of that, so strictly speaking I'm defending the idea not the person.
unless you are can specifically predict an upcoming short term food shortage, it's almost always better from an evolutionary perspective (which is the origin of the instinct wanting a harem in the first place) in an uncertain environment like post-zombie apocalypse to impregnate as many females as possible when you have the opportunity. You never know when another threat might come and depose you so it's better to just impregnate now and then all the girls you impregnate will be roped into protecting your genes (their children) and you're much more likely to be more genetically successful than if you just wait and don't ever impregnate anyone. If you wait years before impregnating anyone then any week an army might come to rescue you and break up your harem , or a stronger armed banned comes and takes your women or zombies might break into your enclosure. His instincts should be to just come inside with abandon.
the reason we don't do that in normal life is because life is so safe and predictable that you can afford to think "oh let's wait until we can afford to pay for our kid's college" , but in a dangerous uncertain environment like a zombie apocalypse the better thing is to just spread your genes to dozens of girls when you have the chance since the chance might not last long.
and yeah from the king's perspectuve he'd most likely just think "creampieing highschool girls feels so good. now that I'm the king I'll do whatever I want."