This question is mostly aimed at guys but I guess there could be a female equivalent.
Would it offend you if your girlfriend made a passing joke about someone having a small penis?
(presuming this is a guy she's never been with and is not seen as being a very attractive man)
A friend made a comment about me having small thumbs, so I said in front of my boyfriend "almost as small as *****'s willy". He started shouting that isn't the kind of thing you make jokes about.
Ok, the joke was crap, but I don't see what's so offensive about it?