Genesis 6:2-4. That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. (KJV)
Job 1:6 Now there was a day when the sons of God came to present themselves before the Lord, and Satan came also among them. (KJV)
Romans 8:14 For as many as are led by the Spirit of God, they are the sons of God. (KJV)
1 John 3:2. Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is. (KJV)
John 3:18. He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God. (KJV)
Jesus is the only 'begotten' son of God. The Hebrew word there is 'monogenes', which means 'only born'. Jesus was God's only child by birth. The 'sons' in Genesis and Job are angels, which were never born but were created directly. In the New Testament passages, Paul and John are referring to people who become sons of God through their following of Him. They weren't born this way, but became children of God through being born-again. Not a contradiction.