The young mother had been looking at her son's dick for a long time and he took advantage of it. When no one else was in the house he easily tricked her into sex. And as I see it, this hungry female didn't mind letting him see her charms. Only she didn't expect him to get that close to her ass so fast. But it was payback for her lust.
Considering how much they'd been drinking, I'm not surprised they had the idea to have a threesome. Especially since the mom is such a badass. Kissing your daughter in front of her boyfriend meant offering yourself up as a pussy for copulation. And the guy took advantage of that offer by banging both of them. He even shared his sperm with his mother when he came between his girlfriend's legs. Damn, that's fair!