Angels are not sexual beings. They do not re-produce
You didn't say, (at least I didn't see it) who are the sons of God in Genesis 6?
The way I have to understand angels in light of Genesis 6 and Jude 1, it may not be that they cannot reproduce, but that they won't in obedience to God. Just as humans can reproduce, but won't in heaven.
Here are twice in the Bible where there is a meeting before God of both angels and fallen angels. It shows that sons of God and the host of heaven, and ministering spirits are all the same - angels.
1 Kings 22:
19 Then
Micaiah said, “Therefore hear the word of the Lord: I saw the Lord sitting on His throne, and all the
host of heaven standing by, on His right hand and on His left. 20 And the Lord said, ‘Who will persuade Ahab to go up, that he may fall at Ramoth Gilead?’ So one spoke in this manner, and another spoke in that manner. 21 Then a spirit came forward and stood before the Lord, and said, ‘I will persuade him.’ 22 The Lord said to him, ‘In what way?’ So he said, ‘I will go out and be a lying spirit in the mouth of all his prophets.’ And the Lord said, ‘You shall persuade
him, and also prevail. Go out and do so.’ 23 Therefore look! The Lord has put a lying spirit in the mouth of all these prophets of yours, and the Lord has declared disaster against you.”
Job 1:
6 Now there was a day when the
sons of God came to present themselves before the Lord, and Satan also came among them. 7 And the Lord said to Satan, “From where do you come?”
So Satan answered the Lord and said, “From going to and fro on the earth, and from walking back and forth on it.”
8 Then the Lord said to Satan, “Have you considered My servant Job, that
there is none like him on the earth, a blameless and upright man, one who fears God and shuns evil?”
9 So Satan answered the Lord and said, “Does Job fear God for nothing? 10 Have You not made a hedge around him, around his household, and around all that he has on every side? You have blessed the work of his hands, and his possessions have increased in the land. 11 But now, stretch out Your hand and touch all that he has, and he will surely curse You to Your face!”
12 And the Lord said to Satan, “Behold, all that he has
is in your power; only do not lay a hand on his
person.”
So Satan went out from the presence of the Lord.