Mary was engaged (KJV: “betrothed”) to Joseph at the time she conceived Jesus:

 26 Now in the sixth month the angel Gabriel was sent from God to a city in Galilee, called Nazareth,
 27 to a virgin engaged to a man whose name was Joseph, of the descendants of David; and the virgin’s name was Mary.
 28 And coming in, he said to her, “Hail, favored one! The Lord is with you.”
 29 But she was greatly troubled at this statement, and kept pondering what kind of salutation this might be.
 30 And the angel said to her, “Do not be afraid, Mary; for you have found favor with God.
 31 “And behold, you will conceive in your womb, and bear a son, and you shall name Him Jesus.
 32 “He will be great, and will be called the Son of the Most High; and the Lord God will give Him the throne of His father David;
 33 and He will reign over the house of Jacob forever; and His kingdom will have no end.”
 34 And Mary said to the angel, “How can this be, since I am a virgin?”
 35 And the angel answered and said to her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; and for that reason the holy offspring shall be called the Son of God. (Luk 1:26-35 NAS)

In other words, God got Mary pregnant while she was engaged to Joseph.

The bible is clear that even if the virgin is only “engaged” to the man, she can commit adultery if she has sex with somebody other than her fiance.  The technical distinction between engagement and marriage disappears in practical life, sexual infidelity in either context is considered adultery:

 23 “If there is a girl who is a virgin engaged (Hebrew: arah, betrothed) to a man, and another man finds her in the city and lies with her,
 24 then you shall bring them both out to the gate of that city and you shall stone them to death; the girl, because she did not cry out in the city, and the man, because he has violated his neighbor’s wife. Thus you shall purge the evil from among you.
 (Deu 22:23-24 NAS)

It doesn’t matter whether God got Mary pregnant by a sex act (as Mormons believe) or without a sex act, the fact remains that He chose to get Mary pregnant while she was engaged to somebody else.

You will say that God is not bound by his own Law, but the Law is supposed to be a manifestation of God’s own righteousness and perfection, so that where God himself violates it, this is just as significant as the human being who violates their own moral code.  In both cases, hypocrisy is a manifestation of imperfection.

And if the idea of God being free to violate his own law be pushed to its logical conclusion, we would have to allow that God is free to assume the form of a man, then rape a little child to death in front of us and then seriously expect us to do nothing since he is not bound by his own law.  No Christian would just sit and watch, frozen up solely because “God is sovereign”, so it would appear that binding God to his own law is best since it creates less trouble than saying he is free to violate it. 
(I argue elsewhere that the bible approves of sex within adult-child marriages).

And in that case, God’s making Mary pregnant constitutes “adultery”, and God cannot escape the charge since he is not free to violate his own law.  Somebody may say that God’s violating his own law is pointless since we cannot do anything about it, but on the contrary, we can note the hypocrisy, correctly classify it as a sign of imperfection, and thus dispense with the “classical theism” view that God is omni-everything.  This would handicap the average apologist who uses classical theism presuppositions to argue her points.