Adriana
Either things turning into something purely sexual with nothing else left. OR one or both of the partners being tempted to (and most likely doing) something that is not good for the relationship. Could well talk about having an affair, some kind of cheating. Probably the 2 combined. Having a meaningless affair which still in the relationship.
Babs
Yeah, I can see this. It's like losing touch with your heart and just acting on other impulses.