A three-line whip to vote in favour of Article 50 would not "force" Labour MPs to vote for it.
It would just be a three-line whip.
MPs would be within their rights to break it, as Jeremy Corbyn did often enough.
And to take the consequences, which should be understood in light of the fact that Corbyn is still alive, is still possessed of all four limbs and all five senses, is still a Member of Parliament, and is now the Leader of the Labour Party.
What would those bemoaning this whip have had instead? A free vote? A whip to overturn the referendum result? What, exactly?