I often fall back to words like assertive or confident, rather than dominant, for that same reason. Sometimes it's the FMC that puts me off. If she's too invested in her own cravings and acts as if the MC didn't matter, I call it a miss. She may well be neither a QoS nor a total slut, but the kind of cuckoldry where the FMC has free pass to follow her attraction to others and the MC is just expected to enjoy it, not my thing.
I do have a guilty curiosity for a scenario where the MC finds his significant other cheating, and immediately discovers he enjoys it, not because he's a loser, but because he's got a grip on his own game and uses the fact that the FMC is oblivious of his awareness of the cheating, to his own advantage. This requires a rather dominant MC, capable of a bit of manipulation, plotting ways to have fun with the situation and, in the end, to reclaim his LI and himself cuck the lover/bull out of the picture. This could be fun to play, alas, not the kind of tropes NTR fans are usually interested in, as it would mean a plot twist that turns the NTR into either NTS or loyal endings. Anyone know a game like this? If an author really could pull an MC capable of reclaiming his LI and who also avoided the tropes of broken marriages, making the NTR entirely of the LI's own doing, and making her a likeable character only through the MC's eyes, where she couldn't otherwise save herself from being seen as a bitch, I'd be down to checking it out, but this is the kind of "ramification" that doesn't get filtered by tags. Certainly would also lend some meaning to a "compassionate" MC