How are other workers likely to react toward employees who choose not to join a union?
If you are talking about a workplace in which most employees join the union but employees are not required to do so. In such a case, the union members are likely to resent the non-union workers. They will tend to think that the non-union workers are being "free-riders." They will feel that their union dues go to create workplace conditions that benefit the non-union workers even though those workers do not pay dues. This is likely to cause friction between union workers and others, which may lower morale and productivity in a workplace.
Since you have this tagged with "human resources," it is worth nothig that this is the sort of issue that management will need to pay attention to in order to prevent problems in the workplace.