There is a lot of evidence that all procreation of any humans in the Bible is done by humans. The Bible has never even ever hinted that humans can procreate with any other creatures of God except with fellow humans. Angels were created for different purposes, they are not human and are not flesh. They don't have human DNA, to say they procreate is to say something that is not found in the Word of God.
There have always been giants in this world before and after the flood. For example, take Robert Wadlow, born in Alton, Illinois, in 1918. At birth he weighed a normal eight pounds, six ounces. At six months, he weighed 30 pounds. A year later, at 18 months, he weighed 62 pounds. He wasn’t fat; he was tall. He continued to grow at an astounding rate, reaching 6 feet, 2 inches and 195 pounds by the time he was eight. When he was 13, he became the world’s tallest Boy Scout. At 13, he was 7 feet, 4 inches tall. Eventually, Robert reached a height of nearly 9 feet! (In comparison, Goliath was about 9 feet, 6 inches.) Robert’s height actually qualified him as the tallest person in modern history. Here are a few videos about him.
Nowhere is it mentioned his parents were anything other than human. Robert was not a supernatural being. He was as human as you and me and so were the giants of Genesis before the flood, the giants of Anak, Goliath and others.
They don't count. You know as a Christian that the Word of God is the truth. It is the inspired word of God written by men who were guided by the Holy Spirit. The Bible says its messages cannot be broken or proved untrue (
John 10:35, 2 Timothy 3:16, 2 Peter 1:21). The Word of God is infallible. Anything, any doctrine that goes against the Bible, that does not harmonise with it is false and should be rejected.
Let us as Christians stick to the Word of God and not to man made doctrine. God bless.