We've got a lot of sociopathic, emotionally-stunted imbeciles out there.
This is worse than being an imbecile. This is outright evil. And the mother is a pig and a loser for trusting this man with her kids. This is why single mothers with young children just simply should not date until the kids are older and able to defend themselves against beasts like this. I know I'll be roundly condemned on this thread for saying that, but no amount of anecdotal "My mom's boyfriend treated us like gems" stories will make up for the many stories of boyfriends who see the kids as competition for their time and thus abuse them or worse, see them as targets for their predatory sexual practices.
And the "mother" of 3 kids, one a 2 month old, who has a "boyfriend", isn't very emotionally mature either, to say the least.