"We are getting married!"
I'm guessing 了in this sentence adds some kind of "finally" to this sentence, some kind of 'completion-feeling'. Can someone deny or confirm? My mother language is neither Chinese nor English so it's hard to explain. ^^
Guess number two: It is perfectly fine to not use 了. Actually, if you want to be literal and interpret the sentence as if the wedding is happening right now (as someone who's not native in english might think), you could remove the 了 and switch the 要 for a 在.