Well, the cheating start with that make-out woman doesn't help... but I'm glad you are thinking about what you want.
- It may be you don't want to explore polyamory at ALL. Just does not float your boat.
- It may be you don't want to explore polyamory with cheating HIM, but might on your own after breaking up.
- It may be you want to work things out and forgive and move past this faux pas and Close and remain monogamous.
- It may be you want to work things out and forgive and move past this faux pas and try to Re-Open in a healthier way later down the road.
- Or maybe you want it to work with this woman in the picture. (rare, but just listing all possibilities...)
Really you have to sort it out for yourself based on what YOU want, what YOU need in a relationship, and what YOUR limits are in a relationship.
Think about what you want your ideal partner to be in your ideal relationship. What's the goal relationship you seek? Living together? Marriage? Closed? Open? Something else?
Then take measure of the man you are with. Can you be happy with him or does he fall short of the mark?