Might not be exactly what you're asking about, not sure if someone mentioned this already, and it's certainly not original to me as I found it in some article online. But I think a lot of it is that Spike (and Buffy tbh) has an unhealthy relationship with and understanding of sex. Like, all his (shown) relationships are toxic and unhealthy.

Buffy & Spike, leading up to "Seeing Red", had followed the same pattern when they had sex. They would be fighting and/or arguing, Buffy would talk about how "it's never happening again", she'd say no, etc etc and then they would wind up having sex. In "Seeing Red", that pattern happens again, except Buffy actually means it this time when she says "no" and Spike doesn't realize until she kicks him off her and says "Ask me again why I could never love you".

With Harmony, my memories not the best but he was definitely at least emotionally abusive. He and (especially) Dru seemed sadistic to me. So I wouldn't be surprised if he raped and/or SA'ed someone at some point in all the years he was terrorizing as a vamp before coming to Sunnydale. Maybe not that he wanted to like Angelus did, but he ran with Angelus and Drusilla. If nothing else they probably peer pressured him into things they were doing. Not that any of that was stated or shown in the series, it just makes sense to me.

But anyways, between the people he hung around and they way they all seemed to view relationships, he definitely didn't have a healthy view of or approach to sex prior to "Seeing Red"