Before you assume anything negative, pleeeaaaseee think about it slower. The way you say it, this looks like the only issue in your relationship. Communicate with him calmly and try to fix this situation. Be honest and tell him how you feel (don't mention other men) and ask him how he feels. This isn't a conversation to make him feel guilty for the "terrible thing he's doing." Because he might be dealing with something internally and externally. And I think as a lover, you should be supportive of him and help him on that before you demand stuff from him. Let us know how it turns out.