The NIV translation of Genesis 6:2 used to say “the daughters of men” but then the NIV translators became “politically correct”, and out of a misguided fear that they would offend people, they changed it to say “daughters of humans”. That is why I will never buy the latest NIV translation.
To say “the daughters of humans” is a mistranslation of the Hebrew, and as your question suggests, causes confusion.
This is the explanation of the expression "sons of God" given in the 2000 edition of the NIV:
The phrase “sons of God” here has been interpreted to refer either to angels or to human beings. In such places as Job 1:6; 2:1 it refers to angels, and perhaps also in Psalm 29:1 (where it is translated “mighty ones”). Some interpreters also appeal to Jude 6-7 (as well as to Jewish literature) in referring the phrase here to angels.
Others, however, maintain that intermarriage and cohabitation between angels and human beings, though commonly mentioned in ancient mythologies, are surely excluded by the very nature of the created order (Genesis chapter 1; Mark 12:25). Elsewhere, expressions equivalent to “sons of God” often refer to human beings, though in contexts quite different from the present one...
“Sons of God” (vv.2, 4) possibly refers to godly men, and “daughters of men” to sinful women (significantly, they are not called “daughters of God”), probably from the wicked line of Cain. If so, the context suggests that Verses 1-2 describe the intermarriage of the Sethites (“sons of God”) of chapter 5 with the Cainites (“daughters of men”) of chapter 4, indicating a breakdown in the separation of the two groups.
What is more important is the warning of the judgment to follow, as described in Genesis 6:3. Whatever it was they were up to, the corruption of humanity displeased their creator.