Marriage
The Sims
On The Sims, only heterosexual couples can get married. Either Sim can propose, but whether the other Sim accepts or not depends on their mood or the relationship between the two Sims. If the other Sim accepts, however, both Sims will change into wedding clothes and share a dance together. Also, if they do not already live together, the Sim who was proposed to will take on the surname of the other Sim and move in with them. There is no special symbol for marriage in the Relationships section.
Same-sex couples can ask one another to 'Move In' if they do not already live together. The other Sim accepting depends on their mood, their relationship or the quality of the other Sim's house.
The Sims 2
Sims can also get a divorce. See Divorce.
When two Sims are in love they can get first engaged and then marry (with or without a wedding party). If the Sim to be married is not living with his or her partner yet they will move in as soon as the wedding ceremony is over. Either Sim can propose, the major difference being, the Sim who proposes is the Sim whose last name both parties to the marriage will take.
There are two ways in which your Sims can get married. First off, both have to be engaged, and can either get married by clicking on the Sim in which they are engaged to and selecting the "Get Married" interaction, in which the two Sims exchange vows and kiss to signify their marriage.
The other way, and more "traditional" way to get married is to have a wedding party. In order to do this, either Sim can click on a telephone, and select "Throw Wedding Party". This is basically like a normal party, only your Sims are expected to get married. You may or may not use the wedding arch. But a difference between normal parties and wedding parties is that any guest you invite will show up even if you have a low relationship with that Sim!
Like normal parties, a wedding party is rated by how good it is. So, it is possible to have a horrible wedding party. In order to prevent this, you can pretty much set up the wedding party like you would a normal party (having music, buffet table, bar, etc.)
Finally, when the marriage takes place, both Sims switch into wedding attire (not to be confused with "formal" attire, since male and female Sims wear black tuxedo and white gown, respectively, instead of their chosen formal wear) and head beneath the wedding arch. Your guests gather around, and the Sims exchange vows and kiss to signify their marriage.
If the "Wedding Party" is successful a limo will come for the couple to take them on their honeymoon.
Same-Sex Marriages
Same-Sex Marriages are referred to as "Joined Union" in The Sims 2, although a Wedding Party is still called a wedding party. A Joined Union is the alternative to a wedding available for Same-Sex Couples.
Like a regular wedding, when two Sims of the same gender are in love they can get first engaged and then marry (with or without a wedding party). If the Sim to be joined is not living with his or her partner yet they will move in as soon as the wedding ceremony is over. Either Sim can propose, the major difference being, the Sim who proposes is the Sim whose last name both parties to the marriage will take.
Sim Unions provide fewer Aspiration points than marriage.