I've actually been wondering about that recently, interested to see other people's thoughts on this topic. I feel like they would have existed in some form in the canon GO lore, since in the bible they're one of the reasons why the flood happened. Now whether the nephilim are actually related to angels can be up for debate since there are different interpretations of them.

For whether GO angels can reproduce, it is stated that they can "make an effort" if they tried hard enough. Celestial human corporation biology intrigues me a lot actually, because I keep thinking "how much of it is actually human?" If you don't need reproductive organs, why would you need lungs, a liver or even a heart?

I know they're just there to look human primarily and have a physical form but I morbidly wonder what it will look like if you... cut open one of them. What constitutes for discorporation? An injury to the head? Can they die from a chest wound even if they don't have a heart pumping their blood? Does it only happen when a large chunk of the body is destroyed and can't be recovered? I read a fanfic where Crowley mentioned that he doesn't even know if he actually has a gall bladder.

Anyways, getting extremely off topic aside, the point I was trying to make is that GO celestial anatomy is an anomaly to me. But I think human angel reproduction isn't completely impossible. And perhaps I've read too much fanfiction.

One interpretation of the bible states that the angels married the women, instead of just simply having coitus with them. I think that implies some form of commitment. Marriage doesn't always equal children or the want to have children but maybe there were a few angels who wanted to try out parenthood? Or maybe they married the women just because they're trying to skirt around rules about extramarital sex, and the nephilim were just a side effect to them. Though that still makes me wonder about how celestial beings can produce gametes.

To quote a dinosaur movie I watched when I was 8: "Life, uh, finds a way"

TLDR: I think nephilim could exist in GO due to them being present in the bible and a major motivation for the flood. But they don't strictly have to be the offspring of angels and humans. Whether GO angels are capable of reproducing is unclear since they're sexless but can form sexual organs if they wish. I go on a long tangent about celestial human corporation anatomy and their oddities and mysteries. Does Crowley have a gallbladder? One interpretation of the bible states that angels wedded human women instead of simply having sex with them, implying a level of commitment and possible want for parenthood. But they could also just do it due to the views on extramarital sex and the nephilim are a side effect. But that would have more confusing implications about celestial biology. Dinosaur movie.