‘find a venue that agrees with your values? I would not try to run a pro-choice ralley in a Catholic church either.’
Catholic churches don’t generally rent out the church for all and sundry to run events in, do they? If they did rent out the church, they would (I expect) enjoy a degree of protection for their religious beliefs, so they would be justified in (for example) refusing to rent out their venue to people who didn’t share their values.
Libraries are public. They are paid for by taxation. They are accountable under the Equality Act, and are not allowed to indulge in belief discrimination.