What is ‘the opposite sex’?

Bellinger v Bellinger (H of L [2003] 2 All ER 593)

The appellent wife was correctly classified and registered as male at birth in 1946, but had dressed and lived as a woman since 1975. She underwent gender reassignment treatment, culminating in surgery in 1981. Later that year she went through a ceremony of marriage with a man, that s11(c) of the Matrimonial Causes Act 1973 provided that the marriage was void unless the parties were ‘respectively male and female’. The wife sought a declaration that the marriage had been valid at its inception and was subsisting.