I don't understand why partisans feel the need to harangue people that want to vote 3rd party. Voters don't owe loyalty to anyone, and if a candidate or party fails to make a convincing case, that's their fault, not the fault of the voters.
There's far too much consternation over this given the miniscule proportion of the vote 3rd parties have typically managed to pick up. More liberals flipped and voted for Bush in 2000 than went for Nader.
That said, anybody who's truly on the left that lets a mean email about Sanders push them toward Trump needs to think about what that really means.
Spoiler: I'm voting for Clinton.
I don't think people say you shouldn't vote 3rd party because you owe loyalty to the Democratic (or Republican for that matter) party.
I might have missed a post somewhere, and lord knows we have a wide spectrum of opinions on GAF, but it is certainly not the main argument people put forward.
I don't want to speak for everyone, or anyone but myself to be honest, so here goes -
I don't think that voting 3rd party can achieve anything. The current voting system will always result in two parties. Yeah, if there was a better party that had a realistic path to replace the Democratic party we could have (and should have) discuss it seriously, but as the things currently stand, voting for a 3rd party is going to achieve nothing at best, and result in a candidate you less like at worst.
And to be clear, I think it's perfectly fine to want to change the voting system in the US so it will allow for more parties, I think it's perfectly fine to want to replace one of the two parties with a better party. If that's a cause you're passionate about, by all means, try to advance that agenda. But I don't think that voting a 3rd party or not voting in the general is going to advance that goal.
If anything, it could hurt it, look at what 2000 did no Nader's political career and the causes he tried to promote (and for the record, I think Nader is unfairly blamed for that loss, even though I think it would've been for the best for him to to try to run in swing states).