I think it was quite arrogant from Theresa May to decide not to involve the Parliament in the first place. People did not give a blanket mandate to her for implementing whatever she thinks Brexit means.
As I understand things, there was a vote on leaving the EU, and the UK voted to leave. And as I understand things, there's only one way to leave the EU, and that's by invoking Article 50.
Seems to me that means there was a referendum on whether to invoke Article 50, and people voted to invoke it. Why should MPs get to block the referendum result?