There are a few arguments for this.
Literal definition: Male and female genitalia, a penis and a vulva.
Negative connotation: calling someone either of the words is not a nice thing to do and both mean the same as calling someone an "a-hole". Further it is not as negative in other countries as it is in north America, in the UK it is pretty prominent.
Equality issues: some will say its a terrible thing because it creates a stigma that women are inferior. In reality banning the word c**t and not D!(k creates a rift that women and men. Like making fun of men and their "parts" is funny or not as big of a deal.
Banning and making a big deal out of "bad words" is not going to help any movement. For example the word Queer is not considered bad but an umbrella term for the LGBT+ community. This was not always the case, It was considered a very mean word for a long time. Other words, not reclaimed, are still around and meaner then ever such as F@g
Calling a man a d**k is roughly equivalent to calling a woman a b**ch. Both of these words are used so often that they are borderline cliche. Calling a woman a c**t is roughly equivalent to calling a man a f**ch guzzling gutters**t. These words are not used as often and are more hurtful to hear.
Also, Dick is the nickname of a lot of men named Richard, which might be the reason it is not banned. We agree that banning words does not help movements.