If the other (non-relationship) sims around really saw them that way in public, they would clutch their pearls and gasp in horror like they do when the Uni EP Streaking scenario is running. They only look that way to us, the player. The sims don't know they are naked, and neither do those around them, because they aren't so it's not really a matter of privacy. What you are describing is probably in part due to the commonly reported timing glitch where a sim fails to do the on-screen spin into a different outfit after doing something where they really should be naked, such as taking a shower or woohooing.
