This is going to be a long post but please stick with me, I've been pondering this over in my head and it'd help to get an opinion. I've been with my girlfriend for almost a year now and there have been a string of issues that make me feel like or relationship will only ever be temporary.  This is mostly due to her sexual views which contrast mine.  She is much more open to sex but stays monogamous to me since I am less liberal about it.  She has made clear though that if we are separated, she still needs physical contact.  Which brings me to the incident...

Last summer we were separated for 2 months due to working in different places.  We agreed before hand that if she did anything with someone else, we would follow boundaries and she'd tell me beforehand.  She did not do anything for 7 weeks but then at a party, she got cuddly with a co-worker and they ended up exchanging oral.  Pretty much they cuddled, he fingered her and she blew him.  Though this bothered me immensely, what hurt me the most was that I was trying to contact her through the party and she never responded until afterwards when she told me she did this with another guy.  It was honestly the worst I've ever felt.

We talked about it and she claimed she didn't consider it sex so she didn't know she should have checked up with me.  But what bothered me was she was outright ignoring me and I saw it as an act of cheating.  However I buried these insecurities and have been with her for another 4 months with no problems.

But I still think about this a lot and I could never see myself spending my life with her because of it.  There's a chance we'll be separated again this summer and I am wondering if it would be an asshole move to give her the option of staying 100% monogamous or breaking up.  Maybe I'm looking for more general advice but I could only think of this sub.  Thanks for the feedback.

TLDR: girlfriend did something id consider cheating.  Wondering if I'd be an asshole to give her an ultimatum of staying 100% monogamous or breaking up