Just because they're not checked for it, doesn't mean the villagers won't breed on their own if they are unsupervised.
Usually it's because they have a lot of skill in parenting (or at least more skill in parenting than in another skill). Plus they are just trying to carry on the tribe (which is sometimes useful if you accidentally forget about your tribe and leave them for a long time).
If you are really bothered about only certain people breeding then my advice is to either
Always pause the game when you're away so you can control the breeding OR make sure everyone has more skill in other areas than in parenting - that means they'll be more likely to choose that skill over parenting.
Of course even with that, accidents do happen
