There could be a few things going on here:
1) Marketo is adding <p> tags around your content when you edit it in the editor. Check out this post: https://nation.marketo.com/t5/Product-Discussions/Rich-text-editor-adding-paragraph-tags/m-p/120996#M79392 for more info on how to adjust the Admin settings to keep this from happening.
2) It might be that there's something happening behind the scenes when you copy/paste into the Rich Text Editor from another source. If you're using the WYSIWYG panel of the Rich Text Editor (the live preview pane) and paste text into the display box, the source styles will be added into the code in the HTML editor.
Could you check the Admin settings, that's the most likely culprit and if you're not able to fix it that way, could you post a before/after of the HTML inside the editable section you're working with here? I suspect there might be a difference between what you're adding into the WYSIWYG editor and what's coming out the other side in the HTML-view.