Sir Elton John has turned down £5.7m for the exclusive rights to his civil union to David Furnish.
Sir Elton John and David Furnish have been together for 12 years
The couple did consider the £5.7m offer from a US television channel and putting the money in the Elton John Aids Foundation, his agent said.
But they told Attitude magazine they had decided to keep the day private.
Their ceremony in Windsor takes place on Wednesday, when civil partnerships become legal in England and Wales.
They have chosen Windsor's Guildhall where Prince Charles married the Duchess of Cornwall in April.
Sir Elton is a globally successful performer
Sir Elton recently said the couple wanted a "very low-key" ceremony.
He said they wanted only their parents as witnesses rather than a lavish event.
He said: "They've been so fantastic to us and so supportive. Out of respect for their support, we want to just keep it small. Not to make a ballyhoo of the ceremony.
"There will be a party somewhere, but the day will be very low-key and we'll take our parents to lunch afterwards."
The first ceremonies under the new Civil Partnerships Act were held in Northern Ireland on Monday. The new legislation means the partnerships become legal in Scotland on Tuesday.