They are indeed fallen angels who had sex with human women.
That is not the case as we can see when we go back to verse 1.
Genesis 6:1 "And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,"
This verse tells us that human men are already multiplying and that human daughters were born to them. So we've already established that human men and women are producing daughters.
Genesis 6:2 "That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose."
So this verse is telling us something completely different. This tells us that the sons of God saw that the daughters of men were fair and took them took them wives. The sons of God are angels. Again, human men are already mating with human women in verse 1. So verse 2 makes no sense if you are believing this is just repeating the same thing.
Genesis 6:4 "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."
We have a second witness to this sin in Jude. And the offspring is certain not normal of human men and women and it's pointing that out.
Jude 6 "And the angels which kept not their first estate, but left their own habitation, He hath reserved in everlasting chains under darkness unto the judgment of the great day."
Jude 7 "Even as Sodom and Gomorrah, and the cities about them, in like manner giving themselves over to fornication, and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire."
What strange flesh? Human women. And they left their own habitation to do so. If you believe differently what fornication did they give themselves or to by going after strange flesh? That is their sin.
I know you believe that the fallen angels are simply spirits and that's it but that is simply not the case. They have bodies and we look like them and vice versa. We can also eat each others food and so on. So if God is telling me that they can mate with human women and that they went after strange flesh I will believe it.