Why is attacking men based on image and insecurity fair game on the left?

Perfect example: the comments in this thread https://www.reddit.com/r/AskALiberal/comments/1it76wn/thoughts_on_the_criticism_of_the_lack_of/

"tiny dick energy" "scrawny" "short man syndrome"

Can't we criticize people for how they are, not physical traits they have no control over? This stuff is pointless and just feeds into how people believe "the left supports hating on men". Hell I feel attacked reading these threads just because I am insecure about my body and apparently being insecure about your body and having trouble dating means you must be a bad person.

EDIT: to clarify, I am asking why it is only men that are fair game. Right wingers are obviously more offensive but at least they are consistently assholes to everyone. Meanwhile the left champions body positivity for women and attacks men with body shaming insults.