"Unified front" That is not the purpose of the union. The union exists to serve it's members. If serving it's members might be slightly more difficult if it actually accommodates the choices of the members it serves, well that's it's job. It's job is not to make it's job easier, it's to serve the members. If the district tries to pressure other members because some make a certain choice, or not provide a safe working environment, That is the fight the union should fight. Not bullying members against making the choice the members feels is the right one to best serve the students.
The possibility of adversarial action by the school district is insufficient to justify the union's actions, especially when, in the case of my school district, the district has otherwise been very responsive to the concerns of the union with respect to health & safety protocols.