I've seen a few series like this, and I definitely prefer them to stuff where the girl either dumps the guy for a better model or plays around behind his back.
I've seen three basic types of reasons/motivations for sleeping with another man: sending a message, study/training, and protection.
Sending a message is the most risky business. The girl wants to get the guy's attention and make him pursue her, or pursue her more seriously, or take their relationships to the next level, etc. In any case, the girl has decided that there's something she wants that her man isn't giving her, and she generally expresses this by deliberately getting it from someone else. Naturally, there's a risk that the girl will suddenly decide to give up on the guy who wouldn't satisfy her and take the guy who would.

Training is far safer, if questionable. The girl may be trying to get experience to avoid pain or awkward situations, or to learn some new tricks or, in rare situations, to achieve some skill her boyfriend wants or even requires. Whichever way it is, she's taking a questionable route for the sake of trying to better please or enjoy her man, not to satisfy some personal desire. She's not unsatisfied with her relationship, and if she's proving her skill she may come up afterwards like a kid who wants a reward for hard work.

Protection is safest relationship-wise, but very dangerous in other ways. Someone is threatening her man, with information, force, or something else, and the girl is putting out to protect someone she loves. These are bad men, and she is doing this because she knows what they can do. She's not likely to stray and fall for one of her assailants, but it's hard to tell just what they'll do to her.

Anyway, I have seen a few works like this, and I'd like to see more. Basically, I'm looking for anything where the girl is being unfaithful for a specific purpose that goes beyond swaying affections or personal desire. Of course, I would like a warning if the ultimate result leaves the guy with nothing but his right hand.