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This is going around reddit today.
It is the incorrect idea that, man is polar opposite to woman, and that man > woman. The next thing to derive from that thinking then is masculinity > femininity. Then masculinity = strong and helpful, femininity = weak and frivolous. Etc, etc.
But in regards to the homophobia comment, it is true I think, a lot of straight cismale homophobia results from some low level knowledge that they have ingrained in their social conditioning an oppositional sexist view of the world.
edit: added links