Pandora's box
the majority of our sex-linked metaphors, expressions, and figures are male-based; female-based ones, like this one, are often negative. However, the familiar and evocative "Pandora's box" is probably the best choice for this meaning. If you need sex-neutral alternatives, consider opening a can of worms, the curiosity that killed the cat, unforeseen consequences, the unknown, mischief, the ills that flesh is heir to, machinations of the devil, all hell breaking loose.
