Most houses I have visited have a cup for drinking water in the bathroom. This makes sense, as drinking with your hands is uncouth, and most bathroom faucets are stubby and horizontal, making it difficult to put your head underneath them without getting shaving remnants all over yourself. You are therefore left to use the cup, which, given who is using it now, what unsavory characters have used it before, and the architectural aesthetic of not building houses with adjacent kitchens and toilets (hindering frequent dishwasher trips), leads to infrequent disinfection.

But none of this explains why I don't like using them.

When I was much younger, my mom, in keeping with the fashion of the time, switched to liquid soap. In addition to being easier to wash with, liquid soap allows small droplets to be stealthily hidden at the bottom of the typically opaque bathroom drinking cup. When the cup is filled, possibly in the middle of the night, you get a stomachful, or at least a mouthful, of nasty soap water.

After being caught in this trap by my younger brother (who also ended up drinking his prank a few times), I compulsively rinse out any cup in the bathroom several times. Even when I lived alone and I had just brought in a freshly washed cup, I found my hands instinctively protecting me.