Since Pinegrow 4.8 (31 may 2018) you can control HTML formatting!
Preserving the HTML formatting
By default, Pinegrow auto-formats the HTML code using common-sense formatting options. That is an issue for users that prefer to keep their own HTML formatting when editing documents in Pinegrow.
Now, the auto-formatting can be disabled in Settings.
With this setting, Pinegrow only auto-formats the elements that are added to the page visually, for example dragged from the Library panel. Of course this also means that you’ll have to take responsibility for formatting the HTML code after manipulating the layout in Pinegrow.
See full release notes v4.8: