Affair

An affair happens when a Sim in love with another Sim interacts romantically with a different Sim.

Getting Caught
Having an affair can be risky as getting caught can possibly ruin the relationship with the other Sim.

Ways a Sim can get caught:
 * WooHooing as another lover passes by on the same lot, even if they're on opposite sides of the lot. However, in The Sims 2, a Sim doing this will not be caught if the other lover is sleeping.
 * Going on a date when the other lover enters the room.
 * A lover sees the flowers a date may have left (though an apparent bug in The Sims 2 means flowers sitting on a shelf or other surface might go unnoticed). However relationship drop won't be as large.
 * A child who sees the adulterers mid-WooHoo may cry and attract the attention of the cheater's spouse.
 * Sleeping in the same bed when the lover is around.
 * Doing any romantic interaction (except light kissing, making out, or proposing engagement) to another Sim while the lover is around.

When this occurs, the Sim will be slapped and many relationship points will be lost.

Interestingly, Sims can be caught cheating by the ghost of a deceased Sim with whom they had a romantic relationship before that Sim died. The ghosts can even become furious with the Sim who cheated on them.

Getting caught by children

If they catch their parents cheating, they get the same relationship decreases with both their parent and the mistress, and if left to their own devices, may even attack their own parent or who the parent is having an affair with. They don't get any visible memories from this aside from any meetings, but effectively act like a Sim who has been cheated on. They'll either cry or get angry at the parent. However, if one of their parents had died and they somehow didn't know about it, it could cause needless drama if they consider looking for love again as cheating.

Jealousy
Whenever a Sim, who has a crush on or is in love with another Sim watches the latter partake in some romantic interaction with another, jealousy occurs. In The Sims 2, if Nightlife or later is installed, jealousy also triggers fury. This doesn't take into account whether or not the feelings are one-sided, which may be messy for unrequited crushes. Some kind of jealous reaction may result if the flirting is done by someone else and rejected, though this is not really confirmed.

The witnessing Sim often loses the crush or love marker, and his or her relationship with the cheater and the one he or she is cheating with plummets into the negative. If he or she doesn't know the other person, a "Met Sim" memory is added along with the affair and possible "Fell Out of Love" memories. This relationship drop enables breaking up, assuming the Sims were going steady, engaged, or married.

The cheated-on Sim will reject the cheater should they try to interact with them romantically.

Repairing relationships
Getting caught in an affair won't always spell the end of a relationship, but it can make bringing it back up extremely hard, especially if the scorned Sim is more interested in fighting with the other one (this could cause the other to lose their own Crush/Love markers, and even trigger Fury to make things that much harder).

To apologize for an affair, select the cheater and click on the spouse cheated on and select Appreciation/Apologize from the pie menu, but only after the fury or "Betrayed" moodlet has ended, if not, the betrayed sim will not accept the apology. It is easier to repair relationships if the Sims are BFF's, if the Sim uses the "Smooth Talk" interaction, or "Repair relationship with..." interaction after studying "Couples Counseling".

If a couple somehow manages to get tarnished by an accidental affair, it may be best to use cheats or hacks, such as InSimenator, to fix the problem, or exit without saving.

As well as repairing jealousy, there is also a way to prevent it. In The Sims 3: World Adventures, a new Lifetime Reward is added; No Jealousy. This reward costs 10,000 lifetime happiness points. Purchasing the lifetime reward eliminates jealousy altogether. The No Jealousy reward also makes it possible to create a polyamorist Sim, with multiple lovers living under one roof.

The Sims, The Sims Bustin' Out and The Urbz: Sims in the City
When cheating occurs, the Sim will be slapped and the 2 Sims will lose -50 relationship points.

The Sims 2 and The Sims Stories
As in The Sims and The Urbz, the Sim will be slapped and relationship points will be lost. In Nightlife, Deluxe, Double Deluxe and The Sims Stories, this can also trigger Fury.

The Sims 3 and The Sims Medieval
When affairs/cheating occurs, the Sim will be jealous and some relationship is lost. No points are lost if the No Jealousy is chosen.

With The Sims 3: Generations, when a cheating occurs Romantic reputation may change, and after sometime the spouse/lover may get the "Betrayed" moodlet and will autonomously accuse the Sim of being a cheater and their relationship will decline.

The Sims

 * Using a romantic social such as Kiss when that Sim has a lover. A sound will also be played if cheating occurs
 * Using an Act Romantic social or the Blastikiss Power Social
 * Giving a hug, even if it isn't romantic, to another Sim while a lover is in the room
 * Dancing with or complimenting another Sim while a lover is in the room

The Sims 2 and The Sims Stories

 * Using a romantic social such as Kiss when the Sim is married or has a lover.

The Sims 3 and The Sims Medieval

 * Using any Romantic Social if you have a lover or husband/wife. No Jealously LTR prevents this effect.