Someone who only wants to stay in or be part of a relationship, whether official or unofficial, for the duration of the honeymoon stage. Once issues come up, they could care less if the relationship ends and they get left.
Mutual Friend 1: "So, whatever happened to Jack and Jill? Are they still talking or are they finally going out? I noticed Jill was ranting on FB and Twitter the other night."
Mutual Friend 2: "Oh, they don't even talk anymore. Guess Jack is a honeymooner. Doesn't know how to deal with problems, big or small, when they arise."