I need help ..

In the beginning of me and my boyfriend dating , he cheated on me by having sex with his ex . Eventually I forgave him and gave him another chance . Recently , I was texting a guy friend (with no motive of any sort) and he began to flirt with me . I fed into it a little bit and didn’t put an end to the conversation right away . My boyfriend found out and now he hates me . He’s disgusted by me , doesn’t want to be seen in public with me , etc . I didn’t have sex with this man ever !! I don’t want to lose my boyfriend but he’s acting like I was talking to this man for months and had sex with him . Even though he actually had sex with his ex and lied to me about it . Now , he’s acting like the world is ending .. but it was supposed to be fine and dandy when I found out about his ex . To this day he wants me to just let it go and move on .. why can’t he just do the same ?