I've seen a lot of girls in my circle, who pretend like they DON"T know certain things, just to make themselves sound so girly and decent. For example, if a topic rises on a simple question on sexual relationships, most of them pretend not to know a lot on that, when we all know that is not actually true.

it's not just about that. Even men pretend not to know the stuff which is considered girly. What is wrong with acknowledging something you actually know?
Have you pretended not to know something because of the embarrassment?