(1)Conduct by a person (A) which would otherwise be an offence under any of sections 16 to 19 against another person (B) is not an offence under that section if at the time —
(a)B is 16 or over, and
(b)A and B are lawfully married.
(2)In proceedings for such an offence it is for the defendant to prove that A and B were lawfully married at the time.