sometimes it is initiated by the girl, but sometimes it is initiated by the man, so it is not about just legitimizing a woman's need to cheat. there are a lot of scenes that have fetishes that are turn-ons to some and turn-offs to others. What turns you on might not turn me on and vica-versa. Cuckolding sometimes pleases one person, but sometimes pleases both in the relationship. Some get off on domination, kink, being in control, etc.... and some get off on submission, humiliation, etc...... Cuckolding, SPH, other BD/SM scenes all have elements of these things, thus, the turn on for some couples.
Some like it and some don't - its all OK.