You have always been able to enter two same-sex Sims into an official partnership. In the first Sims game it was called "Join Union." I believe that's what The Sims 2 called it, as well. In both cases everything was the same except the name. The proposal process, the wedding ceremony, the after-effects (merging households), these were all identical regardless of whether you were marrying a same-sex couple or an opposite-sex couple.
Finally in The Sims 3 we got full same-sex marriage, with "Get Married" being the name of the action you take. And look! The world has not collapsed! In fact, same-sex Sims couples are just like opposite-sex Sims couples, with all the foibles and joys that come along with that.