Translation:We are getting married!
I think "We want to get married!" (rejected answer) should be accepted, but it's a bit misleading compared to "We are getting married!" (official answer) and "We will get married!" (rejected answer)
If two adults said "We want to get married!", I'd interpret it with the same meaning as "We will get married!" If two children said that though, one wouldn't assume that there's any intent to carry it out; in other words, there's a difference between "want" and "will" in that case.
"We are going to be married" is just another minor colloquial way to express it in US English