How do monks transcend the desire for relationships & sex?

Being a 'genuine' monk means a certain kind of dispassion towards the world. How is sexuality & 'relationship craving' effectively dealt within a monk? I am pretty sure a lot of monks struggle with it although they never speak of it. That's probably why many monks disrobe because they can't get the fruits of the noble path. Is it with metta towards the opposite sex or just seeing the unattractiveness of it that craving reduces?