@Confusedmanhurt,
In the first couple of months, a couple is usually not at the exclusive stage (which she pointed out to you). I wouldn't love this but I would probably give her a pass on it.
Edit: I just saw what you added about having the exclusivity talk 4 days beforehand - yeah, that's a lot more problematic. I'd be less inclined to give her a pass but YMMV.
That having been said, have you both been tested for STDs? Because this is how you get STDs - screwing randos.
Consider a few other things about that night:
- Impulsive behavior with no thought as to consequences
- Probably a lot of drinking
- Poor judgment at best
- And bowing to peer pressure
Decide if any of those are deal breakers for you. If they are, then you know what to do.