[TOMT][movie] terrible post-apocalyptic b-movie from the '90s/early 2000's about teens/twenty-somethings who run away from like a cult in a cave to an abandoned city where they meet a couple that seduces them and turns into mutants and tries to kill them

Really bad SciFi movie with no-name actors. At first there are like two guys and two girls, who are either supposed to be in their late teens or early twenties, and they're living in a cave with an older man who's like a father figure/cult leader. He comes onto one of the girls and I think he almost rapes her. Then one of the guys rescues her and the four of them run away.

They wind up in some abandoned city where they meet a couple, a white man with a black woman. They have a really nice apartment and seem cool. Then I remember there's a scene where they're all in a hot tub and the couple are like teaching the kids about sex, because they're really naive or something. Then, later, it turns out they're mutants and they try to kill everyone.

That's all I remember. Thanks in advance for any help anyone can offer!