It's always seemed odd to me that the boy scouts had to accept girls, but the girl guides didn't have to accept boys.

There's (slightly) more context in the linked article. Basically they want to exclude boys because they're called the Girl Guides. They can only do that lawfully under the Equality Act that recently said this was only designed to cover what you were born as. At least that's their logic.

I don't understand why they can't ignore that unless they're being pressured by people who hate trans people on a threat of being sued? Not very clear from the reporting.