Why Socrates believed that sexual desire is the first step towards righteousness

Can sexual desire lead us to something that transcends the physical act? Socrates seemed to think so. In Plato’s Symposium, Socrates recalls the words of his apparent teacher of erotics, the priestess Diotima of Mantinea, who instructed him that lust was the first rung on a ladder leading upwards towards an appreciation of the form of beauty itself and, further, to morality and virtue.