Is talking to other girl and sending dick pic behind your gf back cheating?????

my baby daddy keep brushing it to the side and says he could’ve done worse or people go through tougher shit than this and make it work because once I found out I got up and left with our daughters but I let him say bye to them, he’s basically saying me leaving with the girls was way worse than him doing what he did.

I came to my parents an hour away since I had no one but his family in the town we were living in. He’s making me feel like a shitty mom.. like am i wrong ??? For leaving??