Marriage vs divorce

So my husband and I seperated a few months ago. 2 months ago I moved out. Well this past weekend we spent together. Had a really good time. On Friday I cried right after sex. Anyways. Now were talking about making this work. Meanwhile he has told me he has been with another woman and has feelings for her..myself I been with 1 guy. Well he gave this woman a key to the apartment we shared. I'm so confused if you fall for someone else how can the marriage work after that? Not myself I have no feelings towards the dude i was with he is a friend and that's it. He refuses to change the locks or do anything that would make me feel better. He keeps telling me how much fun we had over the weekend and yes it was fun but I just don't see how the marriage will survive honestly...