Not necessarily. Take, for example, an association that purports to admit anyone who "lives as a woman".
A membership criterion that appears on its face to be fair and neutral may not be so. As was noted by Baroness Hale DPSC in R(Coll) v Secretary of State for Justice [2017] UKSC 40 at [32], [2017] 1 WLR 2093:
“...The question of comparing like with like must always be treated with great care - men and women are different from one another in many ways, but that does not mean that the relevant circumstances cannot be the same for the purpose of deciding whether one has been treated less favourably than the other.”
Also, in Chief Inspector of Education v Al-Hijrah School [2017] EWCA Civ 1426, [2018] 1 WLR 1471, the Court of Appeal considered whether rules in a mixed school by which the pupils were neutrally segregated by sex was direct sex discrimination. Holding that it was, the Court explained that direct discrimination falls to be considered by reference to the position of individual Equality Act 2010 rights holders (in that case, the girls viewed individually, and the boys viewed individually). At [50], Sir Terence Etherton MR and Beatson LJ explained that:
“...The starting point is that EA 2010 s.13 specifies what is direct discrimination by reference to a "person". There is no reference to "group" discrimination or comparison. Each girl pupil and each boy pupil is entitled to freedom from direct discrimination looking at the matter from her or his individual perspective.”
These two cases illustrate that neutral rules do not preclude their treating one sex less favourably than the other and that will be direct discrimination.
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A membership criterion of something like “live as women” allows women to live conventionally for their sex. Generally speaking, this is not going to be a burden for women since all that they are required to do is to carry on living as they always have done within the very broad parameters of whatever “live as women” means. Women are not required to alter how they live their lives in any way.
In contrast, a requirement that men must “live as women” involves a much greater burden. It involves utterly rejecting a conventional lifestyle, it may also involve the use of drugs and/or surgery.
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However, it might well be theoretically possible to come up with a definition that doesn't cause an extra burden on one sex compared to the other.
In that situation it may be that Article 11, Human Rights Act 1998 comes into play. This is the one about freedom of association. Although that has never been tested in court.