The pair are planning an intimate ceremony in May.
They announced their plans to marry last Friday, a day before same-sex marriage became legal in the UK, while celebrating John's 67th birthday in Las Vegas, where the singer is performing at Caesars Palace.
But unlike the $2 million reception they threw to celebrate their civil partnership in 2005, their wedding will be a much smaller affair.
"We'll go to the registry again and we'll take the boys with us," Furnish, 51, told People, referring to the couple's sons, Zachary, 3, and Elijah, 16 months. "Take along the one or two witnesses that we need and make it very small and intimate."
In an interview on " Today," John echoed Furnish.
"We'll do it very quietly," the music legend said about the upcoming nuptials. "But we will do it and it will be a joyous occasion and we will have our children."
John and Furnish have been teaching their sons about how families come in many forms.
"Our oldest is just starting to get his head around daddy and a papa, and not a mommy and a daddy, because he just started to go to pre-nursery school with other kids," Furnish told People.
"He's learning about the mixture of different families. We read books to him that really celebrate the diversity of families in society today. We want to make sure that our boys realize that there are all different kinds of families and all families are great," he said.