Why are short men supposed to only search for shorter women?

It bothers me so much when almost any type of advice to short men is summed up in, "Just go for the girls that are shorter than you".

"Oh you're a short man? At best you can find very short women."

"Oh you wouldn't mind dating a taller woman? Who cares, focus on women shorter than you!"

Why are short men supposed to only search for shorter women?
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