Kakashi's face reveals his teeth, which sparkle so brightly, it mimics the effects of Genjutsu, suspending anyone into a state of perpetual infatuation, even if they are a zero on the sexuality chart. It's not surprising Teuchi-and-daughter were mesmerized.
I once came upon his unmasked face too, but I was lucky, because he was brushing his teeth. Mirrors fortunately deflect light rays, which lose the sensitivity strength it bears. That is Kakashi's face's weakness. Mirrors. If you want to really challenge Kakashi in the shower, bring Link with you.