If a politician always voted with their party no matter what their own personal views it would make a good democracy.
I really whole heartedly disagree with this.
Sometimes democratic decisions can be morally wrong. These are the ones we most need personal, moral convictions for and have individuals prepared to break with the party line.
Progress is usually made by those who stick their head over the parapet in defiance of the current school of popular thought.
We need to also remember that democratic votes are the outcome of only the people who ^actually* vote, and many people don't vote for whatever reason (perhaps they think all options on offer are unrepresentative and morally wrong). The 'majority' might actually only have 30% of the people illegible to vote leaving 70% who effectively abstained or voted against it.
Sometimes it is right to act independently, if its for the right reasons and is done in an appropriate manner.
Whether that includes crossing a picket line or not is open to debate for me.
I'm sure that there are some situations where a democratic vote has been made but individuals feel it is necessary for a few people to cross a pick line in order to maintain public safety for example and they morally feel they can't abide by the majority decision on a single issue but support the union on all other action.
Forcing individuals out of a union as a result is therefore cutting your nose of to spite your face. That person would have crossed the picket line as a non-union member instead. Your union however would be one member down on all other agendas and thus eats away at the bargaining power of the union.
If many people are doing it, it raises questions about how the union have handled the issue and just how representative the union are actually being and whether what they are expecting their union members to do, is a) reasonable b) in their best interests after all
The vast majority won't cross a picket anyway. If you do, you have to have a damn good reason because you know you will get abuse for doing so. If someone is prepared to put up with that abuse, then actually maybe you do need to stop for a second and consider just why they feel so strongly that they have to defy the union.
Its not an easy or straightforward decision to come to and I think perhaps people are too quick to make the assumption that it is.
So no I don't think you should leave the union by default or be thrown out. I think its a very stupid way to deal with the problem tbh. Unions don't 'win' anything or achieve any more power by doing so.