I had the hardest time getting my Sims to marry in Sims 3! I knew that it was POSSIBLE for Sims to get married. I hear people talking about their married Sims all the time! Well a friend clued me in, and it turns out that it just requires a little more patience and better timing.
I have two families that I'm playing. In both cases the couples have moved in with each other and had babies, but I still wasn't getting the "get married" menu option. Why wouldn't my Sims get married?
Back in Sims 1 and Sims 2, once your relationship had hit the "marrying" point, you pretty much had the "get engaged" menu option available all the time. Once you crossed that threshold, your Sims could propose at will.
In Sims 3, the marriage options are a little more elusive! If you want your Sims to marry, the first thing you need to do is get their relationship high enough, obviously. Once that happens (for example, if you have successfully had them "Woo Hoo" or "Try For Baby") you will need to set aside some time (for them) and patience (for yourself).
Start queueing up options from the Romantic sub-menu. Flirt, Kiss, Hug, Backrub, whatever. Mix things up so that you aren't repeating actions too often. I queued up at least 10 Romantic options, then sat back to watch.
In their relationship bar (the one that appears in the upper left hand corner of the screen when your Sims are interacting) wait for it to hit at least "X finds Y Very Alluring." Once your Sim is being found "Very Alluring," it's just a matter of time! And not screwing it up.
Don't push things by queueing up any of the high-risk options like "Woo Hoo" or "Try For Baby." This is not the time for your target Sim to storm off in a snit! Just keep making out and kissing and hugging. It may take a while!
While your Sims are snogging on screen, you will want to keep clicking the Romantic sub-menu to see if any new options have appeared. I spent a solid ten minutes clicking on the Sim, clicking Romantic, looking for new menu options, clicking away (lather rinse repeat).
Eventually, success! The option "Ask to go steady" appeared. (Which is pretty funny, considering that they live together and have two children, but whatever.) I persisted with the Romantic options, and soon "Get Engaged" appeared, and we were set! FINALLY.
You may have to keep clicking for quite a while! At least once I lost the "Go Steady" option through inattention, and I had to wait for it to reappear. I suspect it appears for something like 30 seconds every 30 Sim minutes. Something along those lines. You gotta pounce on it, is what I'm saying!
You may have already noticed that, unlike Sims 2, there is no wedding arch! In order to complete the marriage, you have two options: a Private Wedding, or a Wedding Party. You can hold a Private Wedding just by having one active Sim click on the other. "Private Wedding" will appear on the Romantic menu.
For a public wedding, simply click to throw a party, specify "Wedding Party," and wait for the party to start. When you're ready, have your active Sim click on their fiancée and choose "Exchange Vows." Tres romantic!