Underwear texture "bug"

[Nexing]

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[myuhinny]

That isn't a texture bug the body is using the vanilla body texture which is the body texture that has the painted on underwear you need to install a texture replacer for the male body. 

[Rock&Rule]

I had that issues with hairy underwear trying to mix SoS and SkySight Skins for males. They looked good together minus that issue.  I would check any of the mods that touch male textures.

[myuhinny]

Is your character a custom race that uses their own folder and files? If so then you have to add the texture files manually to their folder as custom races use their own folders separate from everyone else to prevent everyone from using the files and from anything altering the files.