Our fancies are more giddy and infirm, more longing, wavering, sooner lost and worn, than women's are.
In this scene, Duke Orsino presents Cesario with some love advice and attempts to explain that women are not capable of loving like men do because their "hearts lack retention." However, he contradicts himself here, saying that men's love is inconstant and fickle, and that that's why men must marry younger women.