Genesis 6:1-3 Reads:
"1.Now it came about that when men started to grow in numbers on the surface of the ground and daughters were born to them,
2. then the sons of the true God began to notice the daughters of men, that they were good-looking; and they went taking wives for themselves, namely, all whom they chose.
3. After that Jehovah (YWHW or JHVH in the original texts) said: "My spirit shall not act toward man indefinitely in that he is also flesh. Accordingly his days shall amount to a hundred and twenty years.""
Ignoring 1. and looking at 2.:
What the angels were doing was obviously unnatural. In accord with his sentence in 3. God was simply stopping the spread of the Nephlim. However, those "faithful" to him such as Sarah, Noah, and his family were an exception. Why would God limit them?
As found in Genesis 6:8: "But Noah found favor in the eyes of Jehovah." It seems that "Man" included those other than Noah and Sarah. It also seems that Noah, Sarah, and his family did not go under this category:
Genesis 6:5: "Consequently Jehovah saw that the badness of man was abundant in the earth and every inclination of the thoughts of his heart was only bad all the time."
There is also an explanation for why the average life span of humans slowly declined at time progressed. If there was a canopy of water covering the Earth, it would have prevented a lot of UV rays. This would prevent many problems stemming from the sun.
All scriptures above were copied from the "New World Translation of the Holy Scriptures" which goes almost 99.999% unaltered from the copies of the dead sea scrolls and other records of the Bible.
Edited by T Planck, : No reason given.
™ Version 4.2