Hi Op,
I have been in this exact same boat. It took me 4 years though, to start wondering about seeing another man. Not to even up the score, more that I felt that the marriage was broken, so why not.
In my experience, all of the men on those websites will profess that they are in the same boat as you, but 99% of them are absolute players, who will actively seek out women who are emotionally vulnerable, in order to have sex.
I did actually go through with it, with more than one man. Emotionally, it didn't do me too much good in the long run, as I realised I was being played, but at the time I found it very exciting.
I left my H in the end, and am now very happily married to my 2nd husband.
So, I think in summary, I would say : be aware that this man is probably just saying what you want to hear, to get his end away. You won't end up with him, but he could be the catalyst required for change.
You say you can't leave your DH, so I wonder whether there would be any merit in telling him the truth? That you can't get past this infidelity, and won't, unless you also fuck someone else. He might not like it, but what can he say? Even if you don't go through with it, I think it would do him good to wonder what you're up to, when you're on a night out without him.