In Judith Minty's "Conjoined", what does the onion mean?

In the figures of the onion, the twins, and
the calves, the three were intended to be separate, but instead they are freaks of
nature, accidents that allowed them to live but to live abnormally. The speaker suggests
that this situation is like marriage. In lines 12–13, the speaker indicates that freedom
(divorce, separation) might kill one partner, as it sometimes kills a Siamese twin. Note
that the one who might not survive is not specified. The metaphor thus represents
recognition of reality and both a reluctant concession and a decision to adjust. An
increasing number of men now “slice onions,” but men, more often in charge of power and
money, usually lay out the grounds of a relationship. Hence it is often asserted that
women must be alert to hidden dynamics (body language, looks) to protect themselves
emotionally, while men may be more direct and less subtle.