For me there would be a lot more questions.
Was he watching poem and had pop ups or was he on a dating site, did he go looking forward local women? Were they women or men?
Did he close the account befire it after you found out? When did he open it? When did he 'do it'?
Does he seem genuinely sorry to have hurt you or just sorry he's been caught & risks losing you?
Why did he do it? Curious? Horny & you weren't home? Or did he say he wasn't getting enough at home? Quite a different vibe depending on why! (For me).
Humans are visually stimulated, just because he was wanking to porn/live cams/whatever, does not make him some sexual deviant as some are claiming. Doing something sexual outside your relationship - yep. Deviant/creep/dirty - no.
It's ok for this to be something you can accept happened and choose to move on from it, it's also ok to choose that, for you, this is a deal breaker. Just don't be forced (either way) to act in a certain way because of the responses here.
As you get older I think you realise that there's more to life than the mistakes you make. It's all very well at 20 to say 'Nah I'm
Out of here - next!' It's not the same at 50 & many of us want to enjoy our retirement with our partners.
It's not black & white, it's simply which option do you think will make you happiest in the long run (out of the available options! Obviously him not having done it would have been the much better option 💐)
Have you ever been cheated on (that you know if anyway) before? I have & I don't think it would seem/feel the same (depending on some of the answers) but if it did he'd be gone because I'd never put myself through trying to get over it again. But I think wanking over done woman online is totally different, but if it was someone local where the intention was to actually meet up, that's different.
Big Hug. Listen your heart & your feelings. No one elses!!