Gen 6 is taken out of context:
6 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
3 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.
4 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.
5 And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
6 And it repented the Lord that he had made man on the earth, and it grieved him at his heart.
7 And the Lord said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them.
8 But Noah found grace in the eyes of the Lord.
In the early generations of mankind, when he began to multiply on the earth, there was a clear separation and distinction between the Children of God (Seth's descendants ) and the daughters of man (Cain's descendants). However, the male men of God saw the secular woman and found them attracted and married them. (God warned Solomon and the children of Israel not to marry people of the world with good reason.) Intermarrying between the sons of God and the daughters of man removed completely the distinction between the two groups to a point where the sons of God were consumed by Cain's worldly and rebellious generations. Thus, all of mankind was consumed by sinful practices that God wish He had not made man.
As man became more and more wicked God said My spirit shall not always strive with man. In other words mercy will not always plead on behalf of man, especially when wickedness exceeds God's mercy and grace or man can no longer be reached by the God's spirit.
The one hundred and twenty years does not apply to the age of man, but to the number of years Noah preached before the flood waters came and destroyed the wicked antediluvians.
Giants in those days!
Originally, God created man in his image and likeness. Man was made perfect with great height, symmetrical and greater intellect. It is due to the generations and generations of sin and depravity that are form and likeness to God has been marred by transgression. We are from the perfect image and likeness of our Creator. Yes, during the earlier generations close to Adam and Eve men were giants in comparison to succeeding generations and certainly to our generation today.
I agree that Angels can take human form... examples:
II Cor 11:14 "And no wonder! For Satan himself transforms himself into an angel of light."
But angels cannot procreate:
Matt 22:29 "Jesus answered and said unto them, Ye do err, not knowing the scriptures, nor the power of God. 30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven. Clearly an angels existence is not the same as that of human beings. They do not marry nor are given in marriage. There clearly suggests no procreation exist among the angels, no need. Sexual relations has its place within the marriage institution germane to earth. God gave mankind the ability to procreate, not angels. Satan and the rest of the fallen angels hate the fact that God gave man the ability to create through procreation; something they can never do.