According to Ivan Gobry's Martin Luther, Luther thought that
Sin is undefeatable, for lust will inexorably take residencde in each of us, therefore, to condemn oneself to celibacy, intending to please God, is to engage in self-deception and hypocrisy.
Gobry also says that Luther believed the requirement that priests and monks stay celibate to be an unjustified limitation on their freedom:
Why does Rome with such persistence continue to insist that clerics bear the burden of celibacy? Out of greed alone! An unmarried priest bequeaths all his property to the Pope, but one who has a wife and children, will leave the inheritance to them.
(Unfortunately I'm translating from a Russian translation.)