Dude, these recommendations are awesome. I just wish we can have an updated list.
Honestly, this genre just isn't that popular compared to games with cheating wives/girlfriends or harems where all the girls are happy to share the MC so I don't have many more suggestions. That said, there are a few worth mentioning...
Radiant - This game is popular mostly due to the triplet daughters but if you choose to date Brooke, there are a couple of hot cheating scenes. You can take her out to a restaurant for your first date, then bang the waitress in the bathroom and get a blowjob from another girl while you're on the phone with her.
Race of Life - You start off as FWB with your co-worker and she wants a serious relationship. You can give her what she wants... and then cheat on her.
The Sinful City Fight for Love - Not a huge fan of this game for a couple of reasons but your wife is in a coma and you can cheat on her with your ex (who is also married) and some others.
Life of Desmond - Ugly graphics but you can cheat on your girlfriend with a lot of different people including her mother and even her brother if that's your thing.
24 Lustful Hours &
Wicked Ways are focused around the cheating on your wife/girlfriend but quality wise, they are pretty meh. Might be worth a try if you've played everything else and you're really starving for this kind of content.
Edit: There's also
College Bound. You can cheat on your girlfriend with her best friend, the principal, other students, etc. It's completed but a lot of people weren't happy with the ending.