I am constantly getting this issue regardless of whether I take the tags out in HTML option in the WYSIWYG editor. It is a crazy default oftion in Marketo and needs taking out. I sometimes get it if I insert a table, it adds crazy DIVs and paragraph tags.
I am looking for a fix to this which actually works. I have added a style in the header of the HTML of my template to say p {margin:0px} and this does work but has an unwelcome knock on effect when you want to edit the copy.
Any successful fixes out there?