Data knew that his mother was an android because she was using a mathematical progression to impersonate randomized 'blinking" of her eyelids. Does Data count everyones blinks for just in case they've been replaced by an android?
Which begs the question of before his 'emotion chip' what inside him was ever actually random?
After the emotionchip however I can see some erections just completely getting away from him at the most inopportune of times because he can't restrain the SOB. He can't. His penis has the same mechanical torque/lifting power of a towtruck. Elastic briefs are not going to tell his winkle to behave and confirm to the line of his pants by any stretch of the imagination.