Really? I hope you are not basing that on this verse “The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown.” The flood was because of man's wickedness “and that every intent of the thoughts of his heart was only evil continually.”
Do you go to a Church that teaches/preaches topically? Back to the verse, this verse is clear that the nephilim were on the earth in and after those days, which days? The days when the sons of God came in to the daughters of men and they had children, who's children did they have? They had the children of the sons of God and who were those children? They were mighty men of old, the men of renown, not the men of the nephilim.
This verse is so easy to follow, but if (not saying you are) you are taught the Bible using verse from here and there to make a teaching seem Biblical, it is the reason people can read this verse out of context. Reading chapter 4-5 gives the context that interprets chapter 6, there is no mention of any other geneology or generations other then Adam and Cain. If you read Genesis 4:17-24 you the mention of only only daughter being born to Cain's geneology, then with a reading of 5:4, 7, 10, 13, 16, 19, 22, 26, 30, the mention of many daughters and sons born to the geneology of Adam. With these and verses 4:17-24 helps make sense of 6:1, this is the first test from the Lord in the reading of the Bible, it is a contextual test.
The context shows Cain's geneology with only 8 sons and one daughter being born to his geneology and she is not even mentioned as a daughter, she is mentioned as "The sister of Tubal-cain was Naamah." Then you read chapter 5 and there's nine verses with the mention of daughters being born to Adam's geneology, you do not need to be a detective to understand the context only has one geneology that did not have an abundance of daughters being born to them, to understand chapter 6 verse 1 to understand that, "when men began to multiply on the face of the earth, and daughters were born to them," is talking about Cain and his geneology is in the context.
Something to think about.
"ΚΑΙ ἐγένετο ἡνίκα ἤρξαντο οἱ ἄνθρωποι πολλοὶ γίνεσθαι ἐπὶ τῆς γῆς, καὶ θυγατέρες ἐγεννήθησαν αὐτοῖς. 2 ἰδόντες δὲ οἱ υἱοὶ τοῦ Θεοῦ τὰς θυγατέρας τῶν ἀνθρώπων ὅτι καλαί εἰσιν, ἔλαβον ἑαυτοῖς γυναῖκας ἀπὸ πασῶν, ὧν ἐξελέξαντο...
οἱ δὲ γίγαντες ἦσαν ἐπὶ τῆς γῆς ἐν ταῖς ἡμέραις ἐκείναις· καὶ μετ᾿ ἐκεῖνο, ὡς ἂν εἰσεπορεύοντο οἱ υἱοὶ τοῦ Θεοῦ πρὸς τὰς θυγατέρας τῶν ἀνθρώπων, καὶ ἐγεννῶσαν ἑαυτοῖς· ἐκεῖνοι ἦσαν οἱ γίγαντες οἱ ἀπ᾿ αἰῶνος, οἱ ἄνθρωποι οἱ ὀνομαστοί. "
"AND it came to pass when men began to be numerous upon the earth, and daughters were born to them, 2 that the sons of God having seen the daughters of men that they were beautiful, took to themselves wives of all whom they chose...
Now the giants were upon the earth in those days; and after that when the sons of God were wont to go in to the daughters of men, they bore children to them, those were the giants of ancient days, the men of renown. "
I do not see anything about Cain there...
οἱ δὲ γίγαντες ἦσαν ἐπὶ τῆς γῆς ἐν ταῖς ἡμέραις ἐκείναις· καὶ μετ᾿ ἐκεῖνο, ὡς ἂν εἰσεπορεύοντο οἱ υἱοὶ τοῦ Θεοῦ πρὸς τὰς θυγατέρας τῶν ἀνθρώπων, καὶ ἐγεννῶσαν ἑαυτοῖς· ἐκεῖνοι ἦσαν οἱ γίγαντες οἱ ἀπ᾿ αἰῶνος, οἱ ἄνθρωποι οἱ ὀνομαστοί. "
"AND it came to pass when men began to be numerous upon the earth, and daughters were born to them, 2 that the sons of God having seen the daughters of men that they were beautiful, took to themselves wives of all whom they chose...
Now the giants were upon the earth in those days; and after that when the sons of God were wont to go in to the daughters of men, they bore children to them, those were the giants of ancient days, the men of renown. "
I do not see anything about Cain there...