I have heard this complaint a lot, both in person and online.  "My Sims won't marry!"  Marriage used to be such a straightforward business in The Sims 1 and The Sims 2!  Not only is it more difficult for a Sim to get married, with more factors affecting the proposal, but there are also some known bugs in the game which prevent the marriage options from showing up.

Answer 1: S/He's Just Not That Into You

Time was, the marriage and relationship options would just appear on the Romantic conversational menu after your relationship score was high enough (and it had a romantic component).  This is no longer the case!  In order to get there in Sims 3, you have to do what people are calling "spamming with romance."

Once you have established the baseline relationship, start queueing up ONLY options from the Romantic menu.  Nothing else!  No "chat," no "ask about day."  Just straight up Romantic interactions.  

You should start getting some advanced options (like "Woo Hoo" or "Propose Going Steady") once the other Sim finds you "Very Alluring" or "Extremely Alluring."  Keep checking, because those items can roll off the Romantic menu if you're not quick enough!

Answer 2: S/He's With Someone Else

One of the first things I have my Sims do is "Ask If Single."  But even if a Sim is single when you meet them, if you give them a few days they may hook up with another Sim while you aren't looking.  Damn that Story Progression!  

Once you have interacted with another Sim for a while, the "Ask If Single" option will leave the Romantic menu.  You should be able to see if that Sim is involved or not by hovering your mouse over their face on your Relationships panel.  In addition to that Sim's traits and favorites, it will show their marital status.

Answer 3: It's Busted

This is one of those problems which has cropped up, only to be patched, only to crop up again.  What seems to happen is that a Sim will silently have the "married" bit set, even though they are not actually married.  They will happily flirt, make out, and Woo Hoo with your Sim.  But they will never have the "Propose Going Steady" or "Propose Marriage" options show up.

This was happening quite frequently with Sims that you meet in the other countries, if you have World Adventure installed.  Let's say you meet a Sim in China, things go well, and you invite them over after you get home.  Even though they seem to be single, they will never enter into a commitment.  My theory is that this is a side effect of the complex interactions between Story Progression in your neighborhoods, and the lack of Story Progression in the other countries.

I also recently heard about someone who created two adult Sims in Create A Sim.  She added them to the same household, but did not click to indicate that they were either related or married.  However, after playing them for a while, she found that her CAS Sims would never get married.  

Unfortunately, short of restoring your game from a backup to roll back the patch that broke this in your game, all you can really do is wait for the next patch.  And keep your fingers crossed!