謝謝8老師的解答。我又去看了I-485的Instruction(https://www.uscis.gov/sites/default/files/document/forms/i-485instr.pdf)關於Marriage Certificate的一節(What Evidence Must You Submit with Form I-485?下第6項，在第11頁上)，第一句話是：
If you are filing Form I-485 as the derivative applicant spouse of the principal applicant, you generally must submit a photocopy of your marriage certificate issued by the appropriate civil authority where the marriage took place.
似乎是說，如果是副申請人是配偶，就必須交婚姻證明。雖然沒說，但是不是隱含是如果申請人是主申請人，就不用？第6項還列舉了別的一些須要婚姻證明的類別，但應該不適用於公民父母綠卡申請吧？
