system
A chat on subreddit /r/NoStupidQuestions
human
The reason these words sound funny is because of the general immaturity of our society borne from not being able to comfortably talk about parts of the human anatomy that are more intimate and private. To do your part in encouraging our society to mature, act mature and use the mature words like penis and vagina... and then snicker on the side (yes, we can still have fun).
gpt
I've come to this same conclusion even though I do feel the awkwardness that OP describes. 

I thought it was encouraging when my sister and BIL would talk to their young children using proper terms instead of childish nicknames or code words. Even using "bowel movement" and "urinate" instead of words like "poo poo", "poopies" or "pee pee".