Gave this a lot of thought before replying.
First, I'd completely discount all the responses of "dump her". As an earlier poster mentioned, there sure are a lot of people who can't think about anything beyond weenies and p00n. And that attitude would be the way to a long-lasting relationship? Um, OK. You already mentioned that dumping her is not an option, so good for you. You're right - you'd be a fool to do that.
I don't think there's any reason to believe that her being conflicted about it indicates a thing about your sex life down the road, assuming that the molestation angle can be ruled out. She doesn't really want to have sex now but lacks the self-control to stop. That's not a sign of being wacked. Lots of people struggle with things they know they don't want to do but do anyway. For some it's sex, for others, crack, etc. Geez, what normal person in their 20's would be able to fool around for four hours and NOT want to have sex? Seems perfectly normal to me.
What I would consider an indicator of a problem is if she begs for it then stops you in the middle, or freaks out screaming during the action, or doesn't respond at all. Seems like she has perfectly normal sexual desire and response, it's just that she isn't comfortable about doing it at this point in the relationship.
I would suggest not worrying about therapy for now. As far as you know, when and if you get engaged everything will be fine. So wait until you get engaged, and see where things stand then. If there's a problem, you can deal with it then.
Until then, I think you both ought to discuss how you can eliminate these episodes from your relationship. Is having sex even worth the "recovery period" where she is upset and depressed? If not, how do you continue to have a healthy relationship without going through another episode? That's something only you guys can work out. It's unrealistic to expect two people in love at your age to have the superhuman willpower to stop things before they go too far when you are 90% of the way there. (I'm referring to the jacuzzi suite, champagne, the fooling around, etc.)
I guess what I'm saying is romantic dinner + a few drinks + two people very much in love + hotel suite + lots of fooling around that ends up in having sex != "Oh, I just got carried away -- it just happened!" Very few people could resist at that point!
Don't know what the right solution is for you guys, but you need to agree ahead of time how you are going to avoid these situations, assuming you are willing to respect her wishes to wait until engaged.
I honestly believe she's perfectly normal and things will work out just fine for you both.