A few authors popped up with the style of clarkoverns.
You must be registered to see the links
Most of this guy's stories are like this, I think he lost access to his account, then ended up making a new one which he's only submitted a little to but it's sort of the same.
You must be registered to see the links
These stories are pretty interesting but they don't really get into really hardcore graphic sex
You must be registered to see the links
(this guy is great but his only series that has a man getting cheated on are Just One of the Boys and Karen's Nude Day With Coworkers, the others are mostly an inversion with a girlfriend finding out that her boyfriend is being taken advantage of sexually, though these are really good).
You must be registered to see the links
(this guy's Elise and My Roommates story is a lot like Just One of the Boys, with a girlfriend getting increasingly casual with her boyfriends roommates (and in other stories, her own coworkers, girlfriends, girlfriends boyfriends). The difference here is mostly that the girlfriend is sort of ambiguous about things. Boyfriend is sort of "Okay I saw what happened and it was hot but I don't want it going any further than that," having seen the girlfriend flash a guy. Girlfriend thinks he means he saw her let the guy grope her and is like "Okay no further than that, but if he finds it hot maybe I should do it more often." And things escalate because there's only so much edging someone can take.
The big link between most of their stories is that the guy is not being like, utterly humiliated or having his ego destroyed, which I understand can be a sensitive issue. The author of Just One of the Boys actually started writing a side story because he thought it got a bit harsh with the "main" plotline and he wanted things to be more playful.
If these guys have what you like I'd suggest checking on their favorites and expanding outward from there. CHYOA might be an option (for you, or anyone else here) basically being a choose your own adventure with a pretty big archive of stories already. Some have been left to abandon but nothing keeps people from just signing on and adding to them.
All that said, you might try to be a little more specific. Does the man have to know? Or can it all be entirely behind their back? Maybe something that's an "Absolutely Do Not Want" to add?