Honestly, I think that one of JMS' flaws is that sometimes he doesn't explain enough to his readers. I mean, often I like that he doesn't insist on spelling everything out to us as though we were five year olds, but he always chooses the exact wrong times to gloss over things entirely. Canonically, we really don't know exactly how that night went down, other than that it was consensual, that Norman at least was supposedly not in his right mind, and that Gwen both felt sorry for him and found him charismatic. That's not a lot to go on, especially since, well, if there's anyone in the Marvel universe who can function at high levels while being batshit crazy, it's Norman Osborn. *g* I don't find it implausible that a virginal Gwen would end up in bed with Norman, even if she started out trying to comfort him, because it's Norman we're talking about -- he's extremely manipulative and controlling.
I don't think Peter was being portrayed as especially heroic for forgiving Gwen, just as an all-around good guy, but I do think interpreting that comes down to fine nuances in the text, so we'll probably have to agree to disagree.