I think that it all depends on what works for the story, and that when it comes to storytelling sometimes sex is better than no sex. For instance, if you were in an RP where one of the characters were like The Mad Hatter right above me here, and you were exploring what that character would be like in whatever the context of the RP is, then avoiding 'smut' would be prudish and out of character. On the other hand, if you are RPing about a colony of Shakers fighting off a zombie apocalypse, 'smut' [i]could[/i] very quickly hurt what you are trying to do. 

Personally, I think the rules of this forum help quite a bit in moderating the use of sexuality exactly as much as it should be moderated, because I feel that sex-scenes have to pretty much always be treated with a light touch. If this were a more culturally advanced world and we had the enlightened view on sexuality that The Mad Hatter has, it wouldn't be that big of a deal. But since the subject is still shrouded in taboos and old social mores, most people will probably react to sex scenes awkwardly. I don't mean that everyone is prudish, but rather that most people will be quick to see sex scenes as silly and cheap. 

So what I do is this; don't use sex words. No throbbing cockmonsters or wet pussy-cracks. No nut-fucking, or brown starfish, or triple-fist butternut squashing. Not unless you want the scene to seem awkward or silly, that is. If you want it to be taking seriously, however, you avoid porn-words as much as you can while still making sure it is pretty clear what is happening.