Maybe it's a perspective issue - to me at this point voting for the deal is not voting for brexit really. Brexit has happened. This is about approving or not approving this deal. It's not would you like this deal or EU membership, it's would you prefer to have this deal or not have it.
Part of democracy is accepting that you haven't got your preference and moving on to work with what the electorate chose whether you like it or not. I'm a staunch remainer but I'm also pragmatic, this is better than no deal and improvements can be made in the future.
To take a different example. Mr X, an MP, is in favour of the death penalty for murder and rape. A bill comes up in Parliament which will increase sentences for these crimes. He refuses to vote for it even though it would mean they wouldst be on the streets so quickly which is his preference- but he won't vote for it because it's not death. Principled, or cutting off nose to spite face?