I tend to think, the fallen angels that sired the Nephilim, also messed with animals, not just women. I am not positive, but I think weird animals were running around too, animals that God never created. Unicorns, centaurs, Minotaurs, maybe the T rex. There was some off the wall stuff going on. I am pretty sure those weird things were not allowed on the Ark.
And it is quite possible that the sex between fallen angels and human women was not sex as we know it (and in fact might have been beyond human comprehension). Humans often assume that everything in heaven and earth is completely understandable in human terms (i.e., anthropocentrism).;-)
Those half animal/half human legends different cultures have had to come from somewhere.
And maybe it wasn’t just messing with DNA by sex.
Humans tend to presume that *old* means *primitive* and yet Scripture indicates advanced technology. Perhaps that was far more advanced that we give them credit for even to the point of genetic engineering animal/human hybrids.