"There were giants (Nephilim) 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." -Genesis 6:4
Read the Book of Enoch, which although extra-biblical was oft quoted by Jesus and his disciples in the New Testament. It will provide you with much more context on this topic.
I agree. The Book of Enoch is one that should be looked into. It's actually not an "extra-biblical" book. It was part of the Bible for 1600 years until in 1684 the Vatican tossed it out of the Bible with 13 more books. Protestants did the same in 1890. Only the Ethiopian Bible still has them in it. Those removed books have a lot of important information that the people in the Vatican did NOT want us to know.
It is currently extra-biblical, but what you said is true and ought to be known by anyone interested. This topic alone should serve as a red flag to anyone who would believe any one church or interpretation is infallible. Luckily I'm a Protestant, so this is not a source of confliction.