This is a pleasant tactic to spice up sex sessions with both short- and long-term lovers.
It's the bedroom equivalent of our more general seduction role-playing tactic (which you can use outside the bedroom as well).
It works because, well, women are naughty girls who enjoy a dirty man.
Odds are your woman conjures fantasies some of the time while you're railing her.
She may or may not have talked to you about this.
Not all women will. Many fear you'll judge them... or that they'll hurt your pride or make you jealous if they do.
But just like sometimes you might imagine the girl you're giving it to is actually some actress, porn star, or pretty female workmate of yours, or else some other more idealized fantasy woman from a scenario you've cooked up in your head, women do this too.
Rather than stick to your own private fantasies while she sticks to hers, you may opt to have your fantasies join forces... and behold the power of the joint bedroom role-play.