Naruto's the most unpredictable ninja. Of course he'd pull something like this -- the sexy jutsu. Just like during the Kiba fight and he farted. Big deal. Build a bridge and get over it.
Plus, comic relief was needed.
I agree. It kind of feels like people can't even take a joke anymore with this manga. The thing is, while Kaguya may be like a God, she isn't. And part of the point of using the Sexy Jutsu, aside from the comedy and unpredictability of it, was to show that the claim that Kaguya herself formed the history of shinobi and can absorb all jutsu is false. She isn't the sole progenitor of everything nor is she all knowing like a God- she didn't see or predict that jutsu coming, did she?
Also, about the sexist thing.. I think it's understandable that people have strengths and weaknesses when it comes to writing certain things. And just because Kishimoto doesn't meet your standard of what a woman is, it doesn't make him sexist. In fact, he bears the right to do whatever he wants with his story, and if he chose to write a manga with an all male cast, it still wouldn't make him sexist. Perhaps that is just what he wanted to write.