Why couldn't they just as easily made themselves women to have sex with? Smacks of a manmade fable to me.
That's because it is based on ancient mythology. Notice how the account never mentions the angels materializing into women to be able to have sex with men. The WTS has stated that angels are devoid of gender. Why would wicked spirit beings devoid of gender only want to have sex with women? And furthermore, why would a powerful spirit being have sexual cravings, which are something that are purely biological in nature?
Tim Callahan in his book: "Secret Origins of the Bible" writes extensively on this subject. According to Callahan, the evidence shows that these accounts were originally written to be allegorical to depict the human sons of Israel (sons of God) having sexual cravings for the pagan; and much inferior women of the surrounding nations. And so, they forsook their cherished postions and pursued that which was unatural. This Old Testament account has parallels in other Middle Eastern mythologies that predate the book of Genesis.