Each failed attempt increases the chance of the next attempt by half of the base random sex autonomy chance. Partner random trigger attempts to start sex autonomy once every in-game hour by rolling the base random chance. No matter which one, explained below rules apply.
Sex autonomy can happen from any of these triggers: partner randomness, solo randomness, watching sex, watching porn, romance conversation or joining to existing sex. Most of the behavior is altered by the Sex Autonomy Level setting. Unless disabled in settings, Sims will decide to have sex on their own, either with a partner or alone.