<human>: Can you condense the text into three sentences or less?

I [20f] opened my boyfriends [20m] Facebook messages thinking I was logged into my own Facebook and found messages between him and another girl which included her sending him photos of herself naked and them having cyber sex. 

I thought this must be someone else's Facebook because my boyfriend would never say such awful things, he's such an honest, sweet, respectful guy. He says this has been on and off for a year and it was some sort of an alter ego, he says he just did it to see if he still could. He has also never met this girl in real life, they met online.

He admits that he's fucked up and says he's sorry and still wants to be with me but every time I think of him now I can only imagine this awful person is hiding somewhere inside him.

Reddit, should I forgive him or leave him?
<bot>: found nudes and cyber sex conversations in my bf of 3 years' Facebook messages with a girl he's never met, should I forgive him or leave him?