It isn't the shape of the hamburger though really. I get that lots of people don't know of those Hustler images. Even I didn't think immediately of those images, and they are something I have seen lots and read about.
The association I'm thinking of is women = meat. I don't believe that there are many adults who have never come across that association. And hamburgers are made from meat, right?
Why did Hustler present women in this manner in the first place? They weren't the ones who invented the women as pieces of meat insult - it existed already. They just 'played around' with it and took it to an extreme in terms of making no bones about what porn is about. They did it to shock and raise their profile, but they also did it because they are hardcore misogynists (although they think/pretend they are defenders of free speech).
They took the 'women as no better than a piece of meat' thing and actually ground those women up and turned them into burger patties.
They took the insult which started out as steak (bad enough as it is dehumanizing and invokes death) and cheapened it/took it even further by making it cheap disposable fast food meat product that women were being compared to (and they weren't wrong - this is what the porn industry has become like, they need an endless supply of disposable women).
Anyway, be that as it may, I guess it is an obscure reference for a lot of people. The legacy of that reference is the porn industry of today though.
But even people who don't know about the above surely know that in male dominated society;
women = sex
women = meat (and also chattel, livestock, etc)
sex + meat + female = crass.
Perhaps not. Lucky you if you are blissfully unaware. I'm not unaware so it bothers me. Like I say, not that big a deal but interesting to analyse and discuss anyway.