I've wondered about this as well. My own series is full of profanity, but never really gets outright sexual. It's mostly just butt jokes and profane silliness.
Personally, I don't mark NSFW if it's just a matter of a few common swears - ones everyone knows and overhears on the street daily.
I do mark NSFW if the swears are a little more intense or if there are more of them. For example, I used the NSFW tag on one in which my characters were purposefully listing swear words, and one in which they were talking about handjobs. I also used it for one that was about two boners shaking hands, for obvious reasons.
I do believe, though, that when I read through the NSFW policy, it mentioned that the staff have the right to ask you to edit the picture or add the tag, so I assume that if something is a problem, you will be notified and given a chance to fix it rather than suffering any consequences without fair warning.
In any case, as long as you are putting in an effort to make sure your stuff is tagged properly, you'll probably be all right. No one can really fault you as long as you're trying to be responsible about it.