Sugar: sweetness, femininity, etc.
Spice: naughtiness, sexuality, etc.
In far too many cases: the sugar has gone off. It's been laced with poison and has gone rancid. So what you're left with is the spice, the naughtiness and wickedly tasty sexuality. With no leavening whatsoever.
Coupled with a case-hardened ultrasteel bitch, that's a difficult shell to drill through. It takes a diamond-hard bastard to make it through, and he's so inflexible that he cuts up all the gooey softness inside.
Interesting simile or not particularly useful? Either way, there are a lot of poisonous women out there. They've lost their femininity and all that's left is a twisted and fucked-up mess. One that's exceptionally good in bed, though.
The women's goodness and sweetness of yesteryear has curdled, gone sour, then turned all the way to lethal poison. All the men are lining up like ants to take a sip, while she'll never change because the attention from the losers and PUAs are convincing her that this is the right way to go.
Ironically, it's us men's fault - for listening and being gentle with her. We let cupcake get away with bullshit, never corrected her bad behaviour, now both she and we are reaping the poisonous rewards. Men are being used and women are being left lonely.