There are two ways:
1. Get one of the sims and click on the other sim and click propose marriage. Then they will get married.
2. Buy a wedding arch from the buy panel under the party things, place it somewhere and with one of the sims, click on it and choose get married.
You can also have a wedding party before you do either of those things by clicking on your phone going throw party and selecting wedding party and inviting some family and friends. It's really easy to get a good score as long as the people do actually get married during the party and if the party is a success, they are picked up in a limousine and come back with full statistics a few hours later.
Of course if your sims are at college, they can get engaged but not marry so that might also be what happened.
If none of these work there is probably something wrong with your game.
