The line reads thus in most modern translations:
“My little finger is thicker than my father’s thighs.”
The word “thighs” has been translated as waist, back, or even the whole body.
The King James Version and the NASB have the more accurate version: loins.
Simple deductive reasoning here shows that these prideful, rebellious kids were not trying to be modest with their language. This is a classic biblical example of a kid talking trash. Rehoboam was aiming to make his dad look like a wuss, and make himself look powerful. So he put his father down the way any rebellious dude would put a man down. Minify his private parts.
I’ve never known a man to be offended by someone making fun of his thighs…