Monogamous couples have the luxury of needing only to maintain their appeal to a single partner; whereas non-monogamous practitioners repeatedly open themselves to critiques from a variety of potential suitors, which can both bolster and challenge their self-esteem.
The self-image of the ethically-non-monogamous must be resilient and have a foundation of more than just the reflection of one’s current fling. If managed correctly, being non-monogamous can actually help build and maintain a strong self-image even when not being highly “successful” at playing the game.