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I mean really, if I’m a US citizen and I’m pregnant and I travel to Mexico on vacation and happen to give birth....you telling me that my child is a Mexican and not an American citizen???

Of course not, your child is a naturalized US citizen according to 8 USC CHAPTER 12, SUBCHAPTER III, Part I: Nationality at Birth and Collective Naturalization
Via, clause 'c', 'd', or 'g' - depending on the status of the other parent.

(c) a person born outside of the United States and its outlying possessions of parents both of whom are citizens of the United States and one of whom has had a residence in the United States or one of its outlying possessions, prior to the birth of such person;

(d) a person born outside of the United States and its outlying possessions of parents one of whom is a citizen of the United States who has been physically present in the United States or one of its outlying possessions for a continuous period of one year prior to the birth of such person, and the other of whom is a national, but not a citizen of the United States;

(g) a person born outside the geographical limits of the United States and its outlying possessions of parents one of whom is an alien, and the other a citizen of the United States who, prior to the birth of such person, was physically present in the United States or its outlying possessions for a period or periods totaling not less than five years, at least two of which were after attaining the age of fourteen years: Provided, That any periods of honorable service in the Armed Forces of the United States, or periods of employment with the United States Government or with an international organization as that term is defined in section 288 of title 22 by such citizen parent, or any periods during which such citizen parent is physically present abroad as the dependent unmarried son or daughter and a member of the household of a person (A) honorably serving with the Armed Forces of the United States, or (B) employed by the United States Government or an international organization as defined in section 288 of title 22, may be included in order to satisfy the physical-presence requirement of this paragraph. This proviso shall be applicable to persons born on or after December 24, 1952, to the same extent as if it had become effective in its present form on that date;

Your child would be a naturalized (by and act of Congress) citizen of the US from the moment of their birth, but they are not a 'natural born' US citizen, because their Citizenship is based on a law written by Congress.