Why does birth abroad out of wedlock for a US citizen mother result in stricter citizenship acquisition requirements for the child (e.g., 5-year presence) compared to those for a US citizen father?
It doesn’t. The requirements for acquisition of US citizenship for a child born abroad out of wedlock to a US citizen father also requires 5 years of physical presence in the US before the child’s birth, including 2 years after turning 14, as well as additional requirements. So the requirements for being born out of wedlock to a US citizen father are stricter (due to the additional requirements for acknowledgement of paternity and written statement of support) than for being born out of wedlock to a US citizen mother, not the other way around.You are misinterpreting what you are reading from that Department of State page you are reading (although the page could have been written more clearly). It says:A person born abroad out-of-wedlock to a U.S. citizen father may acquire U.S. citizenship under Section 301(c) or 301(g) of the INA, as made applicable by the “new” Section 309(a) of the INA if: […]The phrase “under Section 301(c) or 301(g) of the INA” incorporates the physical presence requirements under those sections of the law, with section 301(c) for birth to two US citizen parents, and 301(g) for birth to one US citizen parent and one alien parent. Section 301(g) requires a physical presence of 5 years of physical presence, including 2 years after turning 14, as described in the “Birth Abroad in Wedlock to a U.S. Citizen and an Alien” section above.If you read the actual law, INA 309(a) says:(a) The provisions of paragraphs (c), (d), (e), and (g) of section 301, and of paragraph (2) of section 308, shall apply as of the date of birth to a person born out of wedlock if• […]And INA 301(g) says:The following shall be nationals and citizens of the United States at birth:[…](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 1 of the International Organizations Immunities Act (59 Stat. 669, 22 U.S.C. 288) 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 1 of the International Organizations Immunities Act, 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,So as you can see INA 301(g) specifies conditions, including the physical presence requirement, to be met for a child born abroad to one US citizen parent and one alien parent, regardless of whether the child is born in or out of wedlock. INA 309(a) says that in the case of a child born out of wedlock to an American father, INA 301(g) will apply only if some additional conditions (relating to acknowledgement of paternity and written statement of support, etc.) are met.