You know, right now, that at some point in the future, you are going to be hurt and disappointed by this guy. Monogamy with him is temporary. It may last for one year or twenty but your instincts are telling you that he is incapable of “forever.” That’s okay; perhaps we all are. The difference is that you know now that it won’t be forever. Does that “knowledge” undercut the benefit TO YOU of monogamy? Only you can say. Good luck.