It's just a different type of being explicit. A lot of rap focuses on violence while a lot of R&B centers on sex. They can both be equally degrading to women; it just depends on what values you put on what etc.
I think a lot of r&b or whatever you call it is overly sexual. It's kind of ridiculous. Like, I saw the video for Rihanna's "S&M" the other day with scenes of her dressed to look all young I guess, all tied up singing about...well, S&M. I guess that's how you keep people interested with those kinds of artists.