Baloney -- read it again.
Heck, I'll post it:
Whereas John Sidney McCain, III, was born to American citizens on an American military base in the Panama Canal Zone in 1936: Now, therefore, be itResolved, That John Sidney McCain, III, is a `natural born Citizen' under Article II, Section 1, of the Constitution of the United States.
If you're going to claim that the plural "parents" automatically implies that the Senate meant that two parents are required, then the Senate must have also mean that being born on an American military base is also required.
The simple fact is that John McCain was born to two American parents, so it said "parents." He was born on a military base, so it said "military base." They phrased it based on his circumstances, not on some grand but unstated theory of natural-born citizenship.