Hi op, I found history on the browser of our family computer for Adultfriendfinder.
When I clicked on it there was a login page.
Luckily for me (or unluckily) my exH had left his email account logged in. So I requested a new password on the site, and got into it when they sent me a link.
At first when I confronted him, he said he was there just to look at porn (which obviously can't be found anywhere else online
)
But I found his profile requesting discreet afternoon meets with women. He'd paid for the gold membership, and there was his Facebook profile photo grinning away. He was a stay at home dad and I was the breadwinner. We lived in a small village and it was perfectly clear who he was if anyone locally was looking. 
Then he denied having the guts to approach anyone. Then I found a letter written in word (it's what he used to do incase he lost a partially written email - write in word and copy/paste it) to someone asking for a meet up because his wife was frigid but he couldn't leave his child.
He cancelled the account and we had counselling but I found him reregistered 8 more times, until I could forgive no more.
I think the mindset of some people is that they have a fantasy and want to enact it, he's tried you and you've said no, but he's fixated on making it happen, convincing himself he deserves it, must try it or some other bullshit and is finding a way.
I experienced this, because I logged into it myself. Childish I know, but I wanted my exH to see how it felt to have your partner on there touting for sex.
What I didn't count on was receiving around 20-30 emails a day from men, mostly married. They all started pretty much with...my wife won't do X,Y or Z.....
So sorry you have to go through this stressful shit, even if it is innocent. But I'd be tempted to login myself if I were you, and see if he has a profile requesting meet ups.