HIM: The Quran just means God didn't have a son like Zeus siring Dionysus with a mortal, through real sex. It's not saying we all can't be children of God with God's Spirit.
ME: On the contrary, the Quran specifically says, "It is not worthy of the Beneficent Allah that He should take to Himself a son. There is no one in the heavens and the earth but will come to the Beneficent Allah as a servant." (19:93) Indeed, the Quran even goes so far as to deny the principle of adoption: "Allah hath not...made those whom ye claim to be your sons your sons. This is but a saying of your mouths. But Allah saith the truth and He showeth the way. Proclaim their real parentage. That will be more equitable in the sight of Allah. And if ye know not their fathers, then they are your brethren in the faith, and your clients." (33:4-5) If Allah forbids men the adoption of sons, how then can you claim that Allah will adopt us as sons, especially when the Quran specifically says that all come to God as servants, never as sons?
HIM: This isn't condemning all adoption at all, but a rebuke to the sinful situation created when Mohammed adopted Zeyd as his own flesh and blood, and then turned around and married his wife, which then violates these Laws:
1) Leviticus 18:8 The nakedness of thy father's wife shalt thou not uncover: it is thy father's nakedness.
2) Leviticus 18:15 Thou shalt not uncover the nakedness of thy daughter in law: she is thy son's wife; thou shalt not uncover her nakedness.
3) Quran 3:4-5 ...nor hath He made your wives whom ye declare (to be your mothers) your mothers, nor hath He made those whom ye claim (to be your sons) your sons.
So if an adopted son is like your own flesh and blood, you may not marry their wives since it is like marrying your own daughter, and also causing the son to have sex with his own mother.