i agree with what shimmer and ladybug originally said.
when i was with my exboyfriend, one of my best guy friends got home from bootcamp and called me while i was at the boyfriend's house. he got so upset, even though he knew this guy was like a brother to me (which means he was never, and would never, be a threat) not too long after that, i was staying with him one night and i decided to go out with a few friends, which he was supportive of when i left. when i got back to his place, he threw a fit about how i spend too much time with my friends and not enough with him (keep in mind, i practically lived with him half the time and i went out with my friends twice during the duration of our relationship) and told me it was better if i just went home. he brought my stuff to me and i left. i'm pretty sure there was another girl there. the next day, he apologized and said it was because he wasn't feeling well. later on, i found tonnes of e-mails and messages back and forth between him and other girls he had been making plans with.
so be aware, jealousy is often a defense for a cheater because it sets you up, that way when you find out about his infidelity, he can counter with "i told you you spend too much time with your friends," etc.