Lucy pointedly not giving a fuck about Mike because he dared to stay with Sandy is perhaps the pettiest she's ever been. Lucy could have a 100% understandable beef with Mike if she felt used and discarded after the night under the tree. But nope, it's all Sandy Sandy Sandy. If Sandy's in the picture, in any way shape or form, Lucy cannot abide it. And remember, Lucy already admitted that Mike was right to hate her, so this isn't about getting Decembered again. It's about not winning the Mike bowl, e.g. her own ego. The girl is poison.