I have read over a thousand romances - contemporary, regency, paranormal, fantasy, and even a smattering of gay and ménage and none of them had cheating except for one book. This was after fourteen books in the series when the author decided a mated guy got bored with his woman and was going to cheat on her after begging the powers that be for her life not one year ago in story time. I binge read that series and once that hot potato got dropped, I was done. Game over. Felt very livid. I'll let someone else buy the next book and tell me if he really cheats before I bother reading it.
Cheating is one of the most devastating things you can possibly do to someone. It completely erodes that person's self esteem and trust. Also, the men I've seen cheat had no remorse and were sorry they got caught, not that they cheated. And one in particular didn't know why it was such a big deal since he said 'sorry'. Like that should be enough. To overcome this and go back to a loving, trusting relationship is hard and trying to make it work in a short romance story isn't going to feel believable to anyone who knows or has experienced a cheater.
In many of those romances the character, typically the guy, tries to get the 'other' out of their head and goes to a bar for a pickup. They may kiss and fondle but they keep thinking about their 'one' until they realize they can't be with anyone else. Even if it was just supposed to be a one night stand, there was something different about that encounter which makes the person balk at doing it again with a random stranger. That is the fantasy - that once you meet 'the one' you are hooked no matter how hard you fight it.