Tasteless jokes with sexist overtones.
I don't understand what so funny about this particular part of female biology and happily most likely will never understand this particular kind of "humor:, if that could be called that word.
Lol well while I agree that people shouldn't be seen as only objects or particular attributes, I don't see a pun or joke about a part of the female anatomy as inherently worse than one about the male anatomy. Crude sexual/body humor has been with us at least since ancient Greek comedy and I'm willing to bet even long before that.
It's funny in part because the physical functions of sexualized bodyparts (giving milk in the case of breasts) are not at all proportional to the value we place on them since, by instinct or culture rather than logic, they're seen as sexually provocative. Boobs are basically milk glands and fat, yet many or most men and some women find them hypnotic and sexually arousing, even though they're usually more a hassle to the person that has them than anything else. This situation is so absurd that it's funny. Hence boobs are funny. Social taboo only further adds to the subversive flavor of the humor.