Downplaying his bs

Lastnight I went out for a few drinks with some girls and I was told that a few weekends ago (the last time my partner had a drink), there was a backpacker chick trying to talk to him, his cousin was moving him along & telling the girl to back up his taken blah blah, only for him to then turn around and continue flirting with his girl and putting his arm around her holding & trying to walk with her. Now I’m beyond pissed because this is the last straw and because he keeps making me look like a damn fool, I told him I’m done i don’t want this shit anymore but he continues to downplay it because ’ I don’t remember’ or ‘I didn’t know that happened’ omgsh I just want to be single!!!!