No it wasn't a crime. It was the smart thing to do really. Because as it turned out he was quite susceptible to his own medicine it would seem. Or maybe he was feeling it as keenly because it was Lucy doing it. Not like he had any intention of testing his reaction to such interactions with anyone else. No this was a role solely fit for his queen~

That being said when she moved on to his neck and even focused on that one spot she had to have known would melt through his reason and restraint if attacked in the right way, Natsu was having a harder and harder time laying still and it only got worse when she shifted against him like that making him keenly aware of what part of her was pressed so firmly against him. If she'd move a little lower she would be able to grasp the full extent of her actions and their consequences.