A new feature of The Sims 3 is attraction, which is available with the latest patch regardless of whether you have Seasons installed.
It's a small mechanic, but it can give your single Sims a romantic target in case you have no other plans.
It seems that for attraction, Sims tend to prefer heterosexuality, but it's not a guarantee based on your Sims' hidden sexual preference numbers.
With all this attraction, new babies, and other lovey-dovey nonsense happening around the Falkon Towers, Becki has decided she wants to try discovering what these human emotions are like as well.
Not willing to go out and actually talk to other life forms outside of her job, Becki decides to set up a dating profile for the Internet.
Attraction has the side-effect of motivating the townies to ask your Sims out on dates somewhat frequently.
Other Sims don't respect your vows, so you may get a lot of calls asking your Sims on a date even if they're married.
There is no way to turn it off (unlike opportunities), so it might get a little annoying if your Sims have particularly active social lives.
Attraction is subject to the same rules as other relationships when considering whether men or women hit on any particular Sim.
Attraction can occur for any number of reasons, though you'll likely see it most often when Sims learn new skills.
When there is attraction between two Sims, each gets a buff if they're in the same room together.