向你要求结婚证的原因是因为孩子自动变成美国公民的其中条件是孩子要在你的legal and physical custody内。看USCIS Policy Manual, Volume 12, Part H, chapter 4。如果孩子的亲生父母还是在婚姻里，那就符合条件。
For purposes of this provision, USCIS presumes that a U.S. citizen parent has legal custody of a child and recognizes that the parent has lawful authority over the child, absent evidence to the contrary, in all of the following scenarios:
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A biological child who currently resides with both biological parents who are married to each other, living in marital union, and not separated;
但是你15年前就离婚了。所以你变成公民（2年前）后不是跟孩子的另外父母结婚的。那就算你拿到结婚证出来也有什么用？这只能证明你离婚之前有你孩子的legal and physical custody。但是这跟你公民的时间没有重叠。所以这不能证明有某一个时间是所有条件同时满足的。
你既然离了婚，legal and physical custody条件就应该用这项满足：
- A child of divorced or legally separated parents where a court of law or other appropriate government entity has awarded primary care, control, and maintenance of the child to a parent under the laws of the state or country of residence.
USCIS considers a U.S. citizen parent who has been awarded “joint custody” to have legal custody of a child. There may be other factual circumstances under which USCIS may find the U.S. citizen parent to have legal custody to be determined on a case-by-case basis.
你应该拿出你离婚后的custody证明。joint custody也行。
