Quote taken from article on Geas on Halopedia:

“The Librarian also imposed a broader geas on the entire human race, which has affected them a great deal throughout their history following the activation of the Halo Array. The geas includes, among other things, a familiarity with Forerunner technology and control interfaces, as well as subconscious instructions on how to gradually achieve more sophisticated technology, including smart AIs and MJOLNIR armor. In addition, as a result of the Librarian's influence on humans, the basic image of what humans view as an ideal female is based upon the Librarian.”

That last part I thought was really interesting. The geas (genesong) she imposes in humanity subconsciously guides them throughout the war (and possibility since the beginning of humanity?). And because of this she’s apparently the ideal female.