Needs more boring legal background

A story from the Daily Mail tells of a couple who married, divorced, and unioned (with intervening sex change). What is not explained is why “They had to divorce after the switch” though they continued living together and being in love. Seems like common law would be fine with them staying married. But it’s a human interest story rather than news, I guess, despite being in a “news” paper. Also unusual is that the subject looks quite nice as both genders.