Why did Nancy forgive Steve even after he called her a slut?

Because this was an age before the notion of “slut-shaming” came into vogue. Nowadays calling a woman a slut might be an unforgivable thing, because in the feminist view you are oppressing women by casting moral judgment on their choices.

But in the 1980’s, the word was more common. It was still not a nice thing to say, but that’s just because to be promiscuous and “easy” was thought of as a bad thing, so being accused of that was bad because you were being accused of bad behavior, not because it was misogynistic or anything.

So while Nancy was angry that Steve was accusing her of cheating on him, it was the kind of thing you could forgive if the person later realized that you weren’t promiscuous and apologized for falsely accusing you. It wasn’t the kind of word that revealed that the person speaking it was fundamentally bad.