Gen. 6

[th1bill]

Gen, 6:4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown.
This is one great example of misusing Scripture. Search the Bible until your fingers fall off, and you will not find Yehovah God committing sin. Why in the world do I throw that in here?
God created Angels to be His servants, and Angles do not reproduce.
Matthew 22:30 ESV
For in the resurrection they neither marry nor are given in marriage, but are like angels in heaven.
In Heaven, we will no longer reproduce, but will be like the Angels who do not reproduce. Let's look for a moment at possibilities.
Sons of God (Biblical Hebrew: בְנֵי־הָאֱלֹהִים, romanized: Bənē hāʾĔlōhīm, [1] literally: "the sons of Elohim " [2]) is a phrase used in the Tanakh or Old Testament and in Christian Apocrypha. The phrase is also used in Kabbalah where bene elohim are part of different Jewish angelic hierarchies.
Men are great at using themselves to relate to anything else, but Yehovah God had the Holy Scriptures recorded and they interpret themselves and that must be accomplished with the aid of Ruah ha'Mashiach, a.k.a. The Holy Spirit.
Mark 4:11
English Standard Version
11 And he said to them, “To you has been given the secret of the kingdom of God, but for those outside everything is in parables,
It is not meant for the Lost Man to understand the parables of God, and they are revealed to the person who submits to the leadership of Yahweh, because those people are indwelt with the Ruah ha’Machiah and He interprets for them.
Colossians 1:26-29
New American Standard Bible 1995
26 that is, the mystery which has been hidden from the past ages and generations, but has now been manifested to His [a]saints, 27 to whom God willed to make known what is the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory. 28 We proclaim Him, admonishing every man and teaching every man with all wisdom, so that we may present every man [c]complete in Christ. 29 For this purpose also I labor, striving according to His [d]power, which [e]mightily works within me.
The simple truth is revealed to those who are the Children of God because they are filled with the Spirit of His Presence. To this end, the Sons of God, spoken of in this passage are the God Fearing, God Worshipping Jews who married the daughters of the unchosen.

 

[B-A-C]

Do not marry not equal do not reproduce.

But even so.. lets go with the two translations you picked, the ESV and the NASB.

NASB
Jude 1:6 And angels who did not keep their own domain, but abandoned their proper abode, He has kept in eternal bonds under darkness for the judgment of the great day,
Jude 1:7 just as Sodom and Gomorrah and the cities around them, since they in the same way as these indulged in gross immorality and went after strange flesh, are exhibited as an example in undergoing the punishment of eternal fire.

ESV
Jude 1:6 And the angels who did not stay within their own position of authority, but left their proper dwelling, he has kept in eternal chains under gloomy darkness until the judgment of the great day—
Jude 1:7 just as Sodom and Gomorrah and the surrounding cities, which likewise indulged in sexual immorality and pursued unnatural desire, serve as an example by undergoing a punishment of eternal fire.

gross immorality, strange flesh (not their own kind). indulged in sexual immorality.
According to these two translations (and others) these angels committed immoral sexual acts.

AMP
Jude 1:6 And angels who did not keep (care for, guard, and hold to) their own first place of power but abandoned their proper dwelling place--these He has reserved in custody in eternal chains (bonds) under the thick gloom of utter darkness until the judgment and doom of the great day.
Jude 1:7 [The wicked are sentenced to suffer] just as Sodom and Gomorrah and the adjacent towns--which likewise gave themselves over to impurity and indulged in unnatural vice and sensual perversity--are laid out [in plain sight] as an exhibit of perpetual punishment [to warn] of everlasting fire. [Genesis 19]

NKJV
Jude 1:6 And the angels who did not keep their proper domain, but left their own abode, He has reserved in everlasting chains under darkness for the judgment of the great day;
Jude 1:7 as Sodom and Gomorrah, and the cities around them in a similar manner to these, having given themselves over to sexual immorality and gone after strange flesh, are set forth as an example, suffering the vengeance of eternal fire.

unnatural vice, sensual perversity, sexual immorality.
It seems angels are capable of committing sex of some kind.

But even today believers and unbeliers havse sex (sons of God? with daughters of men?) .. and they don't produce Nephilim.
So why would that happen then, but not now?

H5303
נְפִל נְפִיל
nephı̂yl nephil
nef-eel', nef-eel'
From H5307; properly, a feller, that is, a bully or tyrant: - giant.
Total KJV occurrences: 3