You make a good point Mum. Your topic has also caused me to remember this as just one of many silly WTB&T Society "interpretations" of a Bible story. In the first place, the original account in Genesis doesn't say this was done by angels but rather by "the sons of God". I don't think any Bible scholar could know for sure just who the sons of God were in this strange context.
Secondly, if they really were angels then God has some serious problems with how He creates spirit beings and boundery issues. To me the story couldn't possibly be true because I am not sexually attracted to insects and yet I am from birth just an imperfect sinner living in the same physical realm as the insects. Since I have no interest in "getting it on" with insects I can't imagine how spirit beings (angels) second only to God, who is said to have created the Universe, would be attracted to smelly ape-like humans from another realm of reality in the first place. Sure, I was programmed to be sexually attracted to females of my own kind and stink. I think human females of breeding age look great but male ants think the same thing of the hormone spurting huge quivery white queen ants ten times the males size. Not to mention just how a "spirit being" angel could have any sexual feelings at all since they are asexual and made up of spirit? The whole concept is absolute nonsense and sounds like a myth or legend created by human beings and not a revelation from God.
If angels could become flesh and "marry" humans is there any indication that the angels were told it was wrong to do so? Why would their offspring be any better than the average human of the time who was supposidely closer to perfection back then according to Watch Tower's own interpretations? How could an angel add "superness" to the genes he is transmitting to the human egg? Chimps share 90+% of our genes but cannot produce chimp-humans so how could an angel add very much extra to the mix and still produce offspring? Wouldn't they have to replicate themselves into as exact of human forms as possible if they wanted kids? Why would sex starved angels even want kids? If I were an angel I'd just make my sperm incompatable and have sex with as many hot young chicks as possible without the complications of pregnancy, especially if I lived for eons and had to watch my children die over and over again.
In short, the whole thing smells of egotistical humans imagining that our chicks are so HOT that even the angels would want them. This crap may have flown with ancient goat herders but anyone with half a brain today should know it is complete nonsense on every level.