How can Jesus be the Son of God?

Firstly, let's be clear about this, Christians do not use the title 'Son of God' to mean that God took Mary to be his wife. At the time of Muhammad it was widely believed that Allah had many sons and daughters. Muhammad, in rejecting this belief, thought that Christians had a similar belief. Christians and Muslims agree that Jesus was born of the virgin Mary, and that Jesus is the word of God and and a spirit from God.

In the Bible we read: The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the One and Only, who came from the Father, full of grace and truth. (John 1:14) "How will this be," Mary asked the angel, "since I am a virgin?" The angel answered, "The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God. (Luke 1:34,35)

Christians use the term 'Son of God' meaning that he is the eternal word of God as stated in the Qur'an (Surah 3:45), having been in existence from the beginning: In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning. (John 1:1,2) In calling Jesus the 'Son of God' we are merely stating our belief that Jesus is God's eternal Word, born of the Spirit of God to the Virgin Mary, to reveal God's likeness (John 14:9) and to save the world from the wages of sin (Romans 6:23 and John 3:17). We believe that through Jesus, God who is Al-baten - the hidden, became Az-zaher - the revealed. No one has seen the Father (al-baten) but the son has revealed him (as being his az-zaher).

A related article 'Jesus: Did he claim to be the Son of God?' is available here.