Well, because if you draw porn... its a little bit complicated. Nothing wrong with the act of drawing the picture, its actually a little bit nicer for anatomy and stuff. Also, you can draw naked people, how cool is that. Actually, you are getting in the territory of a sexual perpetuum mobile, since you can (in theory) satisfy your own porn needs by drawing something. You can supply yourself ;D But porn just draws in people that are just in it for the porn. And I don't always like doing porn. Two of my porn pictures on my tumblr went to 51 notes (both). The highest a sfw pic got was 20-something, and that is only because it was a reaction pic to a follow, and the guy/gal reblogged it.
Also, people hold lower regards for you and insult you with "Your artistic focus lies on porn/You're a porn artist". I get insulted by that, anyways.
So.. it has good sides and bad sides.
Btw, I am drawing Stocking right now and.. it's not really sfw. So, all caution was for nothing xD