The Watchers didn't just marry human women. They did something the book treats as even worse: they started teaching. They handed out knowledge that humanity wasn't ready for.
The ringleader of this part was an angel named Azazel. The book says he taught people how to make swords, knives, shields, and armor — the tools of war. He also taught them how to make jewelry, dyes, and makeup. Weapons and vanity, handed down from the sky.
Other Watchers taught other things: sorcery, spell-casting, reading the stars for omens, the cutting of roots for potions. Basically, they downloaded a whole library of power into a world that had no wisdom to handle it yet.
That's the deep idea here, and it's worth slowing down for. Knowledge by itself isn't evil. A sword can protect. Makeup is just makeup. So why does the book treat this as a disaster? Because it's knowledge given too early, to people who hadn't grown the wisdom to use it well.