If an organisation to which section 12 applies comprises a body the membership of which is wholly or mainly elected, nothing in section 12 shall render unlawful provision which ensures that a minimum number of persons of one sex are members of the body—
by reserving seats on the body for persons of that sex, or
by making extra seats on the body available (by election or co-option or otherwise) for persons of that sex on occasions when the number of persons of that sex in the other seats is below the minimum,
where in the opinion of the organisation the provision is in the circumstances needed to secure a reasonable lower limit to the number of members of that sex serving on the body; and nothing in Parts II to IV shall render unlawful any act done in order to give effect to such a provision.
This section shall not be taken as making lawful—
discrimination in the arrangements for determining the persons entitled to vote in an election of members of the body, or otherwise to choose the persons to serve on the body, or
discrimination in any arrangements concerning membership of the organisation itself.