Data on forcing marketing opt-in?

In a form, if users had to check a checkbox to receive further marketing emails but couldn't continue the transaction unless they checked it, what do you think the drop out rate would be? Does anyone have any data on this? This kind of behaviour is effectively forcing opt-in to continue a transaction.

p.s. I completely disagree with this, but I want some data to back up my argument that it's not a good route to go.