And just like at every other opportunity to do otherwise, Youta learned absolutely nothing. He's remained so comfortable with Lumi's unflagging, immutable love and loyalty, that he takes it--and her--utterly for granted. The moment he realized that he wasn't going to pay the piper for treating her the way he has, he went right back to seeing her as the usual village idiot.
He doesn't realize Lumi can't stop (easily, anyway--that girl's probably gonna survive) because she's been spending the vast majority of her time with Youta as his figurative punching bag: in general and taking all his actions toward her into account, he treats her far worse than any other girl he's ever screwedmet barring the witches--and even that's if I'm not forgetting any extraordinary examples.
All she's ever done for him was love him loyally.
All she's ever gotten in return was abuse at worst and contemptuous pity sex at best--and countless chances to watch other girls (with nearly all of them being inferior to her in one way or another, if not all--from looks to character--but definitely in terms of love and loyalty) be plowed enthusiastically by the guy she loves. Meeting Youta has turned her life into a never-ending NTR story--with her as the cuckquean lead.
So why should she stop? She gets a chance to utterly NTR one random shorthair she got thrown out of a room by her beloved for. She's already best girl, but Lumi's a relative saint if she manages to stop for that shorthair's sake.