What @mdouglas2010 wrote is a very accurate depiction of what happens in the game. Flirty moodlets will lead sims in committed relationships to start flirting with anyone or if they're not flirty but the other sim is (the one they're talking to), they will still accept their advances completely ignoring their own relationship status.
Sims should never autonomously flirt or accept another sim's advances if they're committed in a relationship. (Maybe another story for sims with the player trait)
This is Really Bad for gameplay, please fix ASAP.
A suggestion: add a relationship autonomy setting with options for open or closed relationships. This should be set as closed by default. In a closed relationship sims would never autonomously flirt with others than their spouse and they would always reject other sims trying to flirt with them. If their own spouse saw this flirting happening, they would not get upset at their spouse (who rejected the flirting) but they could appropriately react to the other sim, perhaps depending on their traits and relationship with the "third sim". This setting would mean that even a sim with the Player trait would not autonomously flirt with others if they were in a committed relationship. A lot of players want to be in control of their own storylines (as seen in this thread) but then with the open relationship setting players that desire more chaos in their game could also experience it. It would be great if sims had a social interaction in which they could propose an open relationship with their partner thus setting the relationship as an open one if their spouse agrees. Perhaps a similar system as the Family dynamic could be used to implement this?
In addition to the previous suggestions, perhaps a sim's reaction to flirting could be adjusted so that the player could choose from 1) completely open (will rarely reject a flirt), 2) "opportunistic" (a chance of positive reaction when the Social compatibility is amazing between the two sims, they're in a good mood and NOT in a group conversation) or 3) completely closed committed relationship (sim will always reject flirtation from other sims than their significant other). In this way players could have control over their sims' lives but at the same time they could choose to have some random romantic encounters to create drama in their game. This should work for most if not all play styles.
(edit: fixed missing word, 2nd addition)
