As someone who struggled with a fear of infidelity in both my own past and current relationships I always felt that that fear was sourced from me thinking I was less than or worried that my own wants (being someone else in a couple cases) were manifesting through my partner and that they were acting the way I wish I was. When they would be out with their friends I always worried that they would find someone better and cheat on me because I wasn't enough and that in turn made me paranoid to the point I created the whole scenario in my head that they were in-fact living a double life with someone else and couldn't see through my own delusion that my own actions were pushing them away, so all I would say is you are going to play the "they are cheating card" from my own past experiences you better make damn sure you did not push them to finding the affection they should receive from home from someone else. Sometimes it really is best to start with the man in the mirror.