You definitely do not have to keep seeing her if you don’t want to. If you can’t deal with this, that’s ok.
But in all fairness, I think you did misread things. Some people will disagree with me on this but exclusivity should ALWAYS be discussed plainly and clearly. Assumptions about your relationship status lead to situations exactly like this. Obviously, she has some level of feelings for you. But to her those feelings can coexist with seeing others. You don’t work that way, and that’s just fine. But you guys need to TALK about this kind of thing, otherwise you’ll end up where you’re at now.
I don’t know if she would agree but from your standpoint she was leading you on by making it sound like you were exclusive without specifically saying it. And if you feel that way and you now feel betrayed then it’s best to move on. Also, I’m not sure how I’d recover from seeing the girl I’m dating, exclusive or not, blowing another dude in front of me.
Her roommates HAD to know what they were doing when they let you in and it makes me wonder what they were thinking. This might happen so often that they thought it was either a laugh or justice of some sort.
I’d keep seeing her but not kiss her.