What drives men to cheat even after being in a perfect relationship?

I’ve been in a relationship with my boyfriend for the last 6 years. Honestly he’s the best boyfriend/person ever. I have the most amazing relationship and have nothing to complain.He’s totally obsessed with me 24/7( In a positive way) and he spends the whole day calling and talking to me even if he’s at work. He also puts in lots of efforts to meet me often. We’re practically best friends since day 1.
I often see him counselling his friends not to cheat on their girlfriends and not to do any unethical things in life. He shares each and every detail about his life and also all the things that his friends do, with me. And he also speaks about how bad he feels when any of his friends cheat on their gfs and does some unethical thing.
Even his friends told me lot of times how amazing he is and that he’s the most loyal person of this generation. From the outside he looks like the most perfect person ever.
Buttt one day when i opened his Dropbox to find our old pictures, i saw something that turned my world upside down.
I saw a video call between him and a women who’s a family friend of him. She’s more than 16 years elder to him. She’s married with kids. She showed him her upper body in the call. He had a screen recording of it in his dropbox.
I was in extreme shock after seeing that video. It was a recent video.
My heart dropped and it started pounding loudly.
When I confronted him, he accepted his mistake and cried a lottttt for forgiveness and told me that he has a little pervert side to him since his childhood and it just dint go away so he dint stop her when she was showing him her body in the video call.
He completely blamed her saying that she’s the one who suddenly called and started showing her body.
Now he wants me to forgive him because he says it was unintentional and that he never did anything like this with any other women.
But clearly in the video call, he was smiling when she was showing him her body.
Why are men like this?
What do they get if they do something like this?