Plural, meaning both mother and father are citizens of the USA, right?
The purpose of Article II, section 1, paragraph 5 was to guarantee full and unquestionable unencumbered allegiance to the United States by any and every POTUS. a non citizen parent would not be consistent with that intent. you could not convince me that a child born of an alien parent would have full, unquestionable, unencumbered allegiance to this country. I have seen to many hyphenated Americans waving the flags of their fathers in the streets of America to ever accept such a notion.