I think the core issue is one of anthropology. Official Catholic teaching continues to be grounded in the idea of the human person as a rational animal who, precisely as human, can (and should) rise above one's passions and choose to act in an intelligent, responsible manner, which the Church interprets as acting in accord with official teaching. 
I think contemporary anthropology has moved away from the idea of a rational animal to one that sees reason as but one of the faculties that make us human. In this view, sexual expression is a function on par with rationality -- something the Church doesn't seem capable of accepting.