I also don't see any issue either...
The forum does not stop to promote adult content, the only requirement was the title must have "NSFW" marked and the following rules.
10. No embedded NSFW images anywhere. NSFW content must be marked accordingly.
31. User avatars must not contain NSFL or NSFW content, infringe on copyright or attempt to impersonate a user.
I have seen a campaign previously for an adult site afair, but I forget the name.
Just make sure nothing is visible through the avatar & signature...
