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!
The woman is juicy and very roomy in the body, but she's noticeably happy with sex! It's pretty rare that women of such a large build who have given birth lose such sharp sensitivity in the vagina and only enjoy anal sex so much!