Most of the "smut" I write is more like erotica than smut, I guess, but I figure as far as it goes, it's the same as with all things:

No matter how bad it is to begin with, you can always go back and fix it later.

Many an awful draft has been saved by some extremely thorough revising.

So, just go and write it! Don't stop to think about it or worry about it too much. When you go through and do your revising, you can make it sound a million times better.