Why do some believe simply being a female, makes one incapable of being the president of the USA?

Why do some believe simply being a female, makes one incapable of being the president of the USA?

This question is NOT about a candidates political views. Of course you can and should debate and want/not want a candidate for those reasons.

This question is aimed at understanding why some feel that just the act of being a woman, makes someone incapable of being president. Where does that sexism come from? There are examples from around the globe of women in highest ranking positions (those that have done more than one term as well).

Why do some believe simply being a female, makes one incapable of being the president of the USA?
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