Conduct by a person (A) which would otherwise be an offence under any of sections 38 to 41 against another person (B) is not an offence under that section if at the time—
B is 16 or over, and
A and B are lawfully married or civil partners of each other.
In proceedings for such an offence it is for the defendant to prove that A and B were at the time lawfully married or civil partners of each other.