You reminded me my last project (I was, well... still am, but game dev took almost all my time), a leather chap as accesory, so dick and ass should show there. Modifying the EA leather chaps original mesh in Blender was a royal pain, and then placing the texture in an accessory spot impacted badly on the quality. The usual size for pants on the texture map is big, but for accesories is stupidly small. After a full month I managed to get it done (somehow by the date when the ranch EP was out) but another creator at Loverslab had the same idea as me, and went ahead, so I finished my version but never uploaded it since there was another who did exactly the same.Many creators will have undergarments as attachments. If there is a specific CC you want to wear with the penis, you will need to edit the outfit in Sims 4 Studio. You would need to change from 'bottom' to 'attachment'. If the outfit is composed of several pieces, you might want to specify different attachment points in S4S, Like ring, bracelet etc. WW will remove clothing items during sex but the attachments can remain depending on your settings.
At least years ago I managed to do my other projects, like the leather vest or leather arm band. But this one was a bust