Time to get the message, methinks
You do not ask your customers or prospects if they want marketing email - they don't.
You put the option on your site and you wait for the clicks to tell you about those who do.
This whole situation just might have an interesting effect down the road. One day, we just might see marketing managers be like mother hens : very attentive to how much they contact their (short) list of email addresses lest they provoke yet another enraged Customer to come in and opt out of their marketing. Losing 20 contacts gets them fired.
I like the sound of that.