In the car, Barb disapprovingly speculates that Steve wants to get into Nancy’s pants.

Yes, a thinly disguised narrow mindedness. Boys like Steve are capable of meaningful relationships that, while inclusive of sexual intercourse, are grounded in mutual respect and admiration. I agree that Barb very much embodies contempt and ingrained prejudice against men.

After all, what do these rules teach young women?

That horror movies are no place to learn rules about female sexuality. We'd all be better off if we stopped treating horror movies as supplemental sex ed and sociology instruction.

Should we really accept that the only way to be a moral person is to avoid sex?

And big orgy parties, yes. Barb would agree.

To me, these stereotypes have laughable signs of patriarchal repression written all over them.

I also think so called signs of patriarchal repression are laughable. Societal matrons reliably generate the laughs, though rarely with intention. They struggle to recognize the controlling and abusive tendencies in Barb's relationship with Nancy ("you better still hang out with me" and "you are not this stupid" and "this is not you"), though. It's protective when Barb tries to isolate Nancy from new relationships, wouldn't you agree? Not if Steve said these things of course.

Nobody watches horror movies to learn lessons about sexuality, appropriate behavior, or morality. That's like treating Romeo and Juliet as a guide to conflict resolution. People watch horror movies to escape these ideological impositions. Someone always finds a way to turn an escape from reality into a prescription for changing the attitudes and behaviors of young people. In the context of a supernatural evil deliberately indicating a departure from reality, I must question the motives of anyone who suggests pearls of wisdom hide between the lines of dialog.

This is no different. In a show dealing with the complexities of the Cold War arms race, substance abuse, grief, chemical weapons developed for warfare being re-branded as experimental medical treatments (chemotherapy), and the implications of the decision to authorize experimentation on children (Hopper is broken while Diane has a new husband and baby), as well as physical and psychological torture this has somehow become a representation of one girl's virginity. How shallow and narcissistic must we be to see this show as a statement about the necessary reversal of the depiction of female sexuality in horror films?

tl;dr: The abounding problems are indicative you are barking up the wrong tree.