My own feeling is that, in threads like this, we are in constant danger of falling into the spiritual trap of legalism. Again and again Jesus tells the Jewish authorities off for this. I don't think there's any getting around it, if we spy a nice-looking boy and look twice we are coveting or lusting - or both (does coveting have two t's?). We play with the words in the hope of justifying ourselves but I think scripture is consistently unequivocal, and especially the gospel.
I was thinking yesterday: there is something about the way in which we stand condemned before Jesus that actually saves us at the very same time. It strikes me that the important thing is not to wriggle.
'Let he who is without sin cast the first stone.'
We are united in our guilt.
We are united in our salvation.