The Kiss the Bride hitmaker, 58, 'wed' under a new British law giving official recognition of same-sex relationships and the same employment, pension and inheritance rights as married heterosexual couples.
John is worth an estimated $330 million (GBP185 million).
According to British newspaper The Sun, John told pals, "The whole idea of this ceremony is to show our total trust and commitment to one another.
"If I had asked David to sign a pre-nup I would have destroyed part of that faith we have in one another.
"I just feel that a little something between us would have been lost."