Giants the Bible and Enoch

It's not necessary for the Angels to be sexual beings in order for them to produce the Nephlim.
The book of Jasher has a very short, but very insightful passage that says the sons of the Elohim or their sons had the ability mix the seed of different species

"Jasher 4:18 And their judges and rulers went to the daughters of men and took their wives by force from their husbands according to their choice, and the sons of men in those days took from the cattle of the earth, the beasts of the field and the fowls of the air, and taught the mixture of animals of one species with the other, in order therewith to provoke the Lord; and God saw the whole earth and it was corrupt, for all flesh had corrupted its ways upon earth, all men and all animals."

The book of Jasher is authentic

Jos 10:13 And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies. Is not this written in the book of Jasher? So the sun stood still in the midst of heaven, and hasted not to go down about a whole day.

2Sa 1:18 (Also he bade them teach the children of Judah the use of the bow: behold, it is written in the book of Jasher.)

In essence they were doing genetic engineering. And since we know sperm is not needed to fertilize an egg (it can be done with an electrical charge). The Angels could have very easily produce the Nephlim through genetic manipulation

These hybrid creatures may also be considered transitional species by science.

Jasher 4.18 And their judges and rulers went to the daughters of men and took their wives by force from their husbands according to their choice, and the sons of men, in those days, took form the cattle of the earth the beasts of the field, and the fowls of the air, and taught the mixture of the animals of one species with another�

I wonder if the ancients did the same thing and that pictures of half animal/half human beings were in fact real?

You might find interesting, this passage from the book of Jasher:

16 And all the sons of men departed from the ways of the Lord in those days as they multiplied upon the face of the earth with sons and daughters, and they taught one another their evil practices and they continued sinning against the Lord.

17 And every man made unto himself a god, and they robbed and plundered every man his neighbor as well as his relative, and they corrupted the earth, and the earth was filled with violence.

18 And their judges and rulers went to the daughters of men and took their wives by force from their husbands according to their choice, and the sons of men in those days took from the cattle of the earth, the beasts of the field and the fowls of the air, and taught the mixture of animals of one species with the other, in order therewith to provoke the Lord; and God saw the whole earth and it was corrupt, for all flesh had corrupted its ways upon earth, all men and all animals.

19 And the Lord said, I will blot out man that I created from the face of the earth, yea from man to the birds of the air, together with cattle and beasts that are in the field for I repent that I made them.

http://www.nazarene.net/jasher/

As it was in the days of Noe...

http://www.hope-of-israel.org/bkjasher.htm The Book of Jasher Uncloaked!