"Sons of God were fallen angels and not aliens"

Some have claimed that the Nephilim, or the "sons of God," both mentioned in Genesis 6:2-4, were aliens. This is a wild extension of a common view that the "sons of God" who married the "daughters of men" were fallen angels, and that the Nephilim were products of those "marriages." "Sons of God" is clearly used of angels in Job 38:7 Orig KJV: "When the morning stars sang together, and all the sons of God shouted for joy?". The Septuagint (LXX) here translates "sons of God" as "angels of God." This need not mean that evil angels, or demons, actually cohabited with women.  Jesus made it clear that angels do not engage in sexual activities, at least not angels in heaven. Matthew 22:30 Orig KJV: "For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven."  Nevertheless, evil angels on earth could have used the bodies of ungodly men, by demonic possession, to achieve their evil purpose of producing an evil generation of people (Genesis 6:12).[2]
There are other reasonable suggestions as to the identity of the "sons of God" and the Nephilim. Interestingly, the word Nephilim is only used here and in Numbers 13:33, where it clearly refers to the descendants of Anak, who were big people, but still people. Furthermore, "sons of God" is not used exclusively of angels -- the children of Israel are called "the sons of the living God" in Hosea 1:10 (see also Psalm 73:15; 80:17).

Numbers 13: 32-33 Original KJV: 32 And they brought up an evil report of the land which they had searched unto the children of Israel, saying, The land, through which we have gone to search it, is a land that eateth up the inhabitants thereof; and all the people that we saw in it are men of great stature.   33 And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we  were in their sight. The Anakim: This was another name for the Watchers. It means “the descendants of Anak”, or Enoch, Cain’s son. Though it was said that a flood had been sent to destroy them, there were still entire cities of Anakim in Canaan as late as the time of Moses. And Jewish chronicler Josephus states that even in his own day it was not uncommon for people to dig up gigantic skeletal remains.

A champion named Goliath, who was from Gath, came out of the Philistine camp ... Then the Philistine said, "This day I defy the ranks of Israel! Give me a man and let us fight each other." Reading further, both in 2 Samuel 21:16-22 and I Chronicles 20:4-8, we discover that Goliath had four relatives, also giants:  1) Ishbi-Benob, whose bronze spearhead weighed three hundred shekels (more than seven pounds) and who said he would kill David;  2) Saph (or Sippai), another descendant of Rapha (the giant);  3) Lahmi (the brother of Goliath), whose spear shaft resembled a weaver's rod; and

  4) a huge man (unnamed in the text) with six fingers on each hand and six toes on each foot.
Goliath and these four were all descendants of Rapha in Gath, "and they fell at the hands of David and his men" (2 Samuel 21:22 and I Chronicles 20:8).
David apparently knew in facing Goliath that these other four might also have to be reckoned with. So David prepared himself to take down all five giants, one stone per giant, if need be. The Bible tells us that the other four did fight against David and his army in later battles, and all four were defeated.
David's stone was a bullet of faith, launched from his sling at probably 100 to 120 mph, and with sufficient momentum to break through Goliath's skull bones. The boastful giant never knew what hit him. The text indicates that David ran toward Goliath as he launched his shot, thus adding to its momentum. We can assume from David's success in killing both bear and lion that he was a fast runner (Goliath could never have caught him in open field running) and an accurate shot, probably able to put a stone into a 3- or 4-inch circle consistently from a distance of 20 or 30 feet (an accuracy comparable to that of today's pro baseball pitchers).

Rulers in ancient Egypt and Mesopotamia often proclaimed themselves as "sons of God" to enhance their power and prestige. So, another view with much support is that the "sons of God" were power-hungry rulers and despots, who, in their hunger for power and influence, took many wives in polygamy. They, and their offspring, through tyranny, became "mighty men." (Nimrod was described as a "mighty one" in Genesis 10:8.)

So, there is no need to resort to fanciful suggestions involving aliens to understand this passage of Scripture.

Good Angels do not marry

A contradictory yet enlightening statement about angel marriages was made by Jesus which was reported in Matthew 22:30; "For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven."  Here we read a statement from Jesus saying that angels do not get married.  A possible explanation for this is that the "sons of God"  were the descendants of Seth, and "the daughters of men" were of the godless Cainites.  Remember, God later spoke of the people of Israel as His ‘firstborn son" (Exodus 4:22), and Moses called the people of Israel "children of the Lord your God" (Deuteronomy 14:1).
Therefore, when they took wives it was an unholy alliance between Sethites (God's people) and Cainites (idol worshipers). Also, the word "wives" seems to indicate more that one and therefore a suggests taking more than one wife (polygamy).