In the Book of Enoch, the Watchers (Aramaic. עִירִין, iyrin), are angels dispatched to Earth to watch over the humans. They soon begin to lust for human women and, at the prodding of their leader Samyaza, defect en masse to illicitly instruct humanity and procreate among them.
wiki on Watchers
OK, in that case it would seem that not all watchers defected but only two hundred of them. And that they - or those not fallen - are more usually known as guardian angels.
In that case, one might conclude, the treason was not just hybridisation, but even more betrayal against the human persons entrusted to them.
If a succuba or an incubus were to try seducing a human person, that would not have the privileges of guardian angels to abuse and would have on top of that to reckon with opposition from guardian angels. See how helpless, in such a case, the treason of these 200 would have made their victims?
But then there is a question of when giants were produced and existed. And Genesis 6 verse 4 seems to come in two divergent translations to English.
"since then" vs. "and also afterwards" - punctuation?
When Rob Skiba talks about Nephelim he cites Genesis 6:4 as "and giants were in those days and also afterwards".
If you go to Douai Reims, you do not find the words I cited in the title. Now the Latin Vulgate:
Gigantes autem erant super terram in diebus illis: postquam enim ingressi sunt filii Dei ad filias hominum, illaeque genuerunt, isti sunt potentes a saeculo viri famosi.
Now no words mean "and also afterwards" here. But there are two words that could be a translation from a Hebrew phrase that could also mean that.
"postquam enim" means "since, after that ..."
But in Hebrew possibly postquam (after that) may not have been distinct from postea (afterwards) in all contexts, and either of the two "... enim" (since) or "-que" (and) would probably both indistinctly be "ve/wa" or however it is pronounced.
We could then punctuate two ways.
"in diebus illis. || Postquam enim ..."
"in diebus illis et postea. || ..."
Same Hebrew words would mean "since after that ..." if placed first in phrase concerning Sons of God and Daughters of Men.
Same Hebrew words would mean "... and afterward" if placed last in phrase concerning giants.
This is only what very little I have gathered of Hebrew grammar when ma actually studied it at University. But though it would have been a deliberate choice not to translate "and afterwards" it would have not been a deliberate omission of text, just another choice of translating.
Other question, metaphysical. If angels are pure spirits and spirits can neither lust nor procreate - St Thomas Aquinas' view - the scenario is not quite correct, at least a certain pudicity is present in choice of words. On St Thomas' view fallen angels, demons, can cause conception only in manners analogous to that of doctors using in vitro or more often artificial insemination. If however angelic beings are more like Flash of DC Comics* (suggested in St Athanasius' biography of St Anthony when the latter says that demons "predicting inundations" have only been watching at Assuan and run very fast to Alexandria/Kairo), then angels could have DNA and Nephelim be true or have been true hybrids.
BpI, Georges Pompidou
Sunday after Sts Peter and Paul
*As far as I know, no document of the universal Church, possibly - let's look it up - Stephen Tempier's definitions in Paris - decide for St Thomas and against the other one.
Tempier in Latin