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The objection is understandable as a matter of personal taste: cake is an unusual metaphor for buttocks, particularly if you associate cake with something soft, crumbly, sticky, or liable to fall apart.
However, slang isn't necessarily governed by the literal properties of the object being used as a metaphor. In this case, cake is generally an affectionate or playful reference to a person's backside, often emphasizing its shape or fullness.
You don't have to like the expression, but its meaning is figurative rather than culinary.