Willoughby and Marianne's "love" if you look beyond the surface of open affection is built on an unsteady foundation. Real love should make you more sensitive of the feelings of others not less so. They clearly disregard propriety and the feelings of others. Instead of casually glancing across the room or always standing in the same circles together at parties they disregard the fact that there are even other people in the room. Also their remarks towards Brandon clearly show that they do not bring out the best in eachother, which is another marker that there is something slightly askew in their relationship.