How about Tumblr? It’s respectable enough that mainstream corporations have Tumblrs, while still being very permissive about sex and nudity. Dunno how long that can last, but so far it has. A lot of artists post there.
If you do post your art at Tumblr, and want to participate in the social aspects, make sure to use your main account. Tumblr lets you created as many blogs as you want, but only the main one lets you communicate with other users. Dunno why they do that, but they do.