Single sex clubs, be they golf clubs or any other, are permitted in law. However, it is unlawful for such clubs to admit members of the other sex on unequal terms. They cannot have 'lady members' who are not allowed in the 'full members', male, bars or who are otherwise treated on unequal terms. And, if they admit guests,guests of both sexes must be treated equally.
When this reform was proposed, the instigators soon realised that banning single sex clubs would be difficult to get through Parliament. The law does, in any case, allow establishments which exist to promote the interests of one sex only and which are defined by gender; male voice choirs, for example. Distinguishing where a club was not so identified was not always easy; some St James's Clubs were created exclusively for men, and their original constitution declared so, though we might not think that the gender was essential to such an establishment now.