Was it an original idea of theirs? Or was it about the legitimization of an already existing custom? For example, Heinrich Bullinger's father was a married dean already in 1504 due to an unofficial sanctioning by the bishop of Constance (see Wikipedia article).
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.)
God said in Genesis 2:18, "It is not good for the man to be alone. I will make a helper suitable for him."
Paul said in 1 Corinthians 7:1-2, "Now for the matters you wrote about: 'It is good for a man not to have sexual relations with a woman.' But since sexual immorality is occurring, each man should have sexual relations with his own wife..." (Ever notice how many times on the news, priests get in trouble with this?)
Zwingli and Luther both had a great understanding of the Bible, and reformed many of the catholic church's laws. So they probably found these verses and acted on them.