The WI weren't being forced to change their policies. The WI had choices.
The WI had to decide whether to
a) follow the law and legislation that exists, and has done for a long time, that their organisation was already supposed to be following but weren't, and correct their policies to reflect the law.
or
b) stick with their moral beliefs, get rid of the single sex exemption they were using, and inform the charity commission they were changing what the organisation stood for and were now a mixed sex group. And if they lost their charitable status then that's a price they would be willing to pay to stick with their morals.
The WI could have continued allowing women and men into their organisation but they would have had to change some things to make that happen.
The WI instead chose to keep their current charitable reason for existing. And the WI chose to continue using the single sex exemption.
The WI ultimately chose the option that will continue to benefit them the most in the long term, which was to tell the men they were no longer allowed to be a member of the WI.
Morals be damned.