Also the bodily function part. It's okay to eat in public (bodily function, everyone does that).
It's okay for a baby to fill a stinky nappy (guess they can't help it). And presumably we all know babies are filling nappies with wee all the time.
Breathing is okay, coughing or sneezing within limits.
Even burping can be laughed off on occasion.
I don't see why expressing milk should be so different, but in many people's minds it is. Unfamiliarity I suppose.