Invisible Vanilla clothing is a commonly known issue when using Robert's Male bodies. I couldn't yet gather any information as for why it is invisible, but the cause is rather simple and easy to fix.
As a matter of fact in Oblivion your body comes with your clothes. It doesn't matter the slightest which body mod you're using, when you're wearing Vanilla items you will have a Vanilla body mesh underneath.
The skin texture however is assigned per race, no matter what mesh is currently trying to use it. Of course Robert's mod makes it so its own textures with their unique layout are used for the races. However, those will not work properly on Vanilla body meshes. Apart from the texture layout not matching and weird skin sections being at off places on the body due to this, this becoming invisible thing also seems to be quite common, although yet still unexplained.
Anyways, the easy fix is to make sure you're not using any Vanilla body meshes anymore, by always using the proper Stock Clothing and Armor Replacers for your respective body mods.
Strangely it was always missed in the documentation of body mods, but those are actually far away from "optional". Just consider a custom race with no texture support for Vanilla... it will be completely purple or pitch black or invisible (the game engine seems to decide this very unreliably) when wearing items from the original game.
As for the invisible underwear I can "see" in this screenshot, make sure the textures it's pointing to are actually at their proper place. There were some changes between Robert's V3, V4 and V5 and mixing up their meshes and textures can lead to this result. And here you see an example of the unreliability of the game engine's choice of error indicators mentioned above. Usually "missing textures", indicated by purple meshes, has priority over "missing normalmaps" (".._n.dds"), indicated by either pitch black meshes or completely invisible ones (somehow depending on the slot the item is worn in and some other factors yet unknown to me in detail), but as you can see this is not "always" the case.
Edited by DrakeTheDragon, 17 April 2011 - 09:07 AM.