"It’s frequently claimed that if a man and a woman are friends, it’s because one of them must want to have sex with the other."

Who exactly is saying this if it's 'frewuently claimed'?

You seem to have based your whole piece on the spurious idea that it's women who are claiming this. While I've seen this question many times, it's often (if not more) from men.

With regard to the idea that women think every man wants to screw them - this probably comes from the fact that women and girls are constantly told to moderate how they dress so that they don't distract the boys and men. We are policed all the time about 'modesty' and 'decency' so if we get the idea that that's all men think about - it's no surprise really.