Bragging about your dick often, kind of makes you a Dick
People that complain about bragging are the same people that complain about some people making more money. I don't mean literally the same people, but qualitatively the same: lacking those things and thus wanting to minimize the ability for people that have them to use them.
A man shouldn't brag about his penis for the same reason he shouldn't brag about his salary, his car, or anything else. Because it's unbecoming of a gentleman.