你不需要之前的結婚證，而是需要custody證明

向你要求結婚證的原因是因為孩子自動變成美國公民的其中條件是孩子要在你的legal and physical custody內。看USCIS Policy Manual, Volume 12, Part H, chapter 4。如果孩子的親生父母還是在婚姻裏，那就符合條件。

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:

• 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年前就離婚了。所以你變成公民（2年前）後不是跟孩子的另外父母結婚的。那就算你拿到結婚證出來也有什麽用？這隻能證明你離婚之前有你孩子的legal and physical custody。但是這跟你公民的時間沒有重疊。所以這不能證明有某一個時間是所有條件同時滿足的。

你既然離了婚，legal and physical custody條件就應該用這項滿足：

• 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.

你應該拿出你離婚後的custody證明。joint custody也行。