I find it hard to believe that so many have never heard derogatory words used for men.
That's plainly not the case. This is an interesting aside, nothing to do with Deep vs. Heard, but it does tell us a great deal about the broader society surrounding it, the discourses of which are clearly feeding into perception of it.
What I and others have suggested isn't that derogatory words don't exist for both sexes. They plainly do. The issue is that words used for female anatomy words are viewed as far more taboo and profane than those used for men.
Dick/prick are probably on a par with twat. They're mild profanities and sort of funny. Compare 'cunt'. How many times have you heard someone say 'I never use that word!' or awful situations arise when 'cunt' is the only expletive deemed low enough to use? It's largely seen as one of the most taboo profanities in our language.
Now compare language used to describe a promiscuous person (note, also, the original etymology of some of those words). Women: slut, whore, harlot, slag, Jezebel, tart, slapper, trollop, floozy, tramp, scrubber. Those are just the ones that immediately spring to mind.
Comparative adjectives for a man? Cad, rake, player, stud. There are significantly fewer, but it's the tone that is so noteworthy here. There are a lot more censorious, derisory connotations associated with the former list than the latter.
Obviously, people can only use what language is available and understood in those contexts to start with. But the fact that the above is true - and it is true, there are serious studies into it by linguists such as Deborah Cameron - says all you need to know about the way in which men and women are viewed in society in general.