What pod are you on? I agree with BeHai, I'd open a support ticket. I'm still seeing the option to send a text only version. so this may be a pod or instance specific bug.
Sorry about the confusion! The Trigger field was added to offer folks the ability to test their Email Script(s). For more information, please check out the “Testing Your Script” section of the Email Scripting developers page: http://developers.marketo.com/documentation/email-scripting/
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