I've had the same problem as well. It all boils down to the fact that the "regular" SOS schlongs are set to male only.
You could easily modify this in CK, but I am not very comfortable in CK yet. I've done a few minor edits and personal "player home" mods, but nothing like what these guys do.
Anywho, open up the actual schlong in CK and you will see the male and female previews for it. Just make sure that it appears on both and uses the correct schlong graphics for the females. I believe that is all you need to do.