No. He went and slept with someone as soon as you guys were "done." He knew things were not sorted out. Out of respect for the relationship, he should have waited. He clearly had her in mind and didn't feel anything to go out and sleep with her. You should have stayed with your gut and stay broken up. Complete disregard for you.

I would not get back with someone after this. Sex isn't a casual thing. It means a lot for me to open my heart and life to you. For him to throw your years away like that without being sure everything was completely over is ridiculous.

Some people think technicalities make things morally acceptable. I completely disagree. He knew what he was doing and the shame you saw was him getting caught, not that he did it.

The lack of respect for you is ridiculous.

You need to find someone else.