converting to coupledom?

What happens when you convert a relationship from the way it began to something new? I ask because I heard a funny story recently:

A couple began their relationship as “friends with benefits,” and then became a legitimate couple. The male half of this couple bemoans their lackluster sex life, proclaiming that the fun is gone now that he can no longer say things like, “Get over here and suck my cock, bitch!”

While I’m going to decline to delve into whether or not it’s reasonable or appropriate to speak like that to a woman, I’m not entirely certain I agree that such things necessarily have to change. We’re all turned on by different things, and there’s no reason a couple can’t continue to be playful, filthy-minded and mouthed, or to engage in role play.

Still, I can certainly see how “normalizing” things could lead to routine and feelings of monotony.

I’ve not personally lived through one of these shifts, so I’m curious to learn whether any of you have stories. Please share your perspective with a comment.