Updated 8/28/13 alterations made to align better with episodes up to and including "Red Team III"
They don't get married because Will told them both separately that they were perfect for each other and to just get on with it. They both know he only said it because his wife was bugging him to say something to them that would lead to a break in the awful routine they were in. In actuality he had already asked for her father's blessing over a month before and she'd already found the ring.
They don't get married right after Charlie tells them that their behavior as a couple thus far both on and off air gives him complete faith that this will work out and cause no problems. He says that he's proud. He tells anyone that will listen to him for the next few months that his work daughter just got engaged and how the couple makes nerds look good.
They don't get married in Vagas during that conference they went to that one time. MacKenzie suggested it drunkally two nights before they left. They do nothing but laugh in her face every second they get before they leave. They never tell her that it actually almost happened.
They don't get married in a chapel. They don't want to. She planed the perfect wedding with someone else once. She's just not going to do that again, even after Maggie and most of the women in the office offer to help. He's always been a little skeptical about the spectacle that particular kind of wedding causes and doesn't think that it's the best way for them to start their marriage.
The do get married on a lark. They have five witnesses, none of whom are parents or siblings, although some of them might as well be. Jim only mentioned in passing that his friends had decided not to get married at city hall that day, and that there was an opening if they wanted it. He swears he meant it as a joke the moment he realizes that they are taking the offer seriously, and he swears it again, every ten minuets until the six of them, Don, Sloan, Will, Mac, Jim, and Maggie, arrive at city hall. Charlie arrives minuets before they go in with a bouquet of white carnations. She wears Gucci and her glasses. He wears a plaid shirt, jeans, and her father's old wrist watch, the one the man gave him when he asked for his blessing. He cries and she doesn't. They go to Hang Chew's after and then they do the news. It's very them.