What happens when Viola, in disguise, goes to Olivia on Orsino's behalf?

In disguise as Cesario, Viola goes to Olivia to plead for Orsino's love. However, Olivia falls in love with her in her disguise as Cesario. Cesario returns to Orsino, telling him that she wasn't successful. He sends her once more to Olivia, who finally declares her love to Cesario, who leaves, saying only "never to love any woman."