What main reason does Higgins give for not marrying?

Shaw sets up a strange, almost Freudian symmetry between Higgins and his mother on the one hand and Eliza and her father on the other. Higgins gives one of his reasons for never marrying as his too great respect for his mother. Her love of beauty, art, and philosophy has led her son to value Milton's poetry and his own universal alphabet more highly than he could a relationship with a woman.