Conduct by a person (A) which would otherwise be an offence under section 25 or 26 against another person (B) is not an offence under that section if—
the relation of A to B is not within subsection (2) of section 27,
it would not be within that subsection if section 39 of the Adoption Act 1976 or section 67 of the Adoption and Children Act 2002 (c. 38) did not apply, and
immediately before the relation of A to B first became such as to fall within section 27, a sexual relationship existed between A and B.
Subsection (1) does not apply if at the time referred to in subsection (1)(c) sexual intercourse between A and B would have been unlawful.
In proceedings for an offence under section 25 or 26 it is for the defendant to prove the matters mentioned in subsection (1)(a) to (c).