I had to learn to remember this within the dream and to forcibly wake myself up from it to stop this happening. Which I think is quite an accomplishment, if I may say so myself.

I suspect it goes back to wearing nappies/diapers as a baby. Babies naturally try to wait until they are unclothed to relieve themselves; they essentially have to learn how to go in a nappy, or else let it happen because they can't hold it in any longer. Then they have unlearn this with potty training. It's crazy really.