Some here confuse Nephilim with their fallen angels fathers.Nephilim were the offspring of fallen angels and women.
Why they didn't materialise as women? Good question, God describes himself as a male and so it seems that his angels do too. The Bible never says that any of them materialised as women and there is no account in any of the apocrypha of such materialisation.
International Standard Version
some divine beings noticed how attractive human women were, so they took wives for themselves from a selection that pleased them.
(Sounds like the women had no choice and were forced to be part of this.)