You don't have to pay on many of these sites; you just register and make yourself a free profile.
I have been through/am going through something similar. I too know far too much about these horrible, marriage wrecking sites. My H registered himself as 'unspecified' re marital status and specified (from a drop down list) that he was 'just' (!) interested in 'discreet online chat'. (There was also some nasty info he had entered below about the type of sex he likes; nothing shocking but SO devastating for me to find; and SEE how he was 'averstising himself'). I alternately torture myself over this or reassure myself that he has no intentions of actually meeting internet slags for cheap nasty shags.. but the damage is done. As has been sad below, trust gone. I don't know what the future holds. He swears he hasn't been back on these sites in months but there have been many instances of me finding out he has been looking at porn, which, sadly for him, I used to tolerate without a problem, even laugh about, but now, with each reoccurance, I just get ill with stress and shock that he has done again what he swore not to, and intense worry about what it will lead to. (Those sites again or worse, and me having to instigate us splitting up. Because I have been SO miserable in recent months because of all this that it crosses my mind sometimes that I couldn't possibly more miserable without him. And I would be learning to live again without all this horrible horrible stress and fear that never really goes away. I have told him that he is risking losing his life as it is for one where he is free to say and do what he wants with internet tarts and whoever so he needs to think very carefully about what he does in future but I fear that he has a problem; an addiction of sorts, or at least a compulsion. I do hestiate to split the family up though; it seems selfish in a way as he is a decent father and the childred would be so upset.. and then I think how rich to see MYSELF as selfish! HE has done this!)
I can shed no real light onto why 'happily' married men do this but I have discovered that men think very VERY differently from women and are able to separate love and sex in a way that must women don't and can't. They compartmentalise this little 'harmless hobby' of theirs and think that what we don't know does not need to hurt us or affect us in any way. Many of them are very simple creatures in this respect.
I suggest that you go back and scrutinise his profile(s) and see what he has regsitered himself as 'wanting'.. it usually says on one of the pages. If he is looking for online thrills only (like my shit of an H) than this is slightly different from looking to meet people offline. And you may be able to get through this.
It's funny, I describe my H as a shit too.. but if I didn't love him I wouldn't still be going through this. I do love him but the trust is gone.
Counselling is a very good idea.
My heart goes out to you. I read these type of threads a lot on here and it's so sad that there are so many but it gives me a feeling of being supported by all the women in the same position.
I hope you can work this out. Please keep posting.