I think it's a combination of things
1. The gods curse his fertility because that's their thing, but they curse him slightly less with his, you know, actual wife.
2. It can't just be any woman, it has to be a special selection. So it's not an unlimited selection.
3. The public knowledge of his 'infertility' would be politically damaging, thus it can't be done blatantly
4. He can't just have children anywhere, they'd be picked off by political adversaries
5. He actually has a number of children. Given how he decided to kill his dad, maybe he felt he now had enough on his plate to manage as any of his children, if not managed right, might manage to kill him. The more children the more his capacity to manage is used up - it may well run out and how could he let something get out of his grasp, eh?
6. The time frame for the apocalypse means children of a certain age are simply of no use, thus its time to stop breeding.