Besides the obvious -- seducing someone you've invited over or are living with -- there are additional situations built into Kinky World.
*Note that your sim is likely to refuse the invitation unless at least a bit horny.
Pizza delivery - order a pizza, and the delivery guy may ask for a little reward.
Repairman - Use the "Call for Repairs ($50)" option on a broken object (toilet, TV, sink, shower ...). If aroused, your sim may flash the repair guy, then be asked for a little woohoo.
*Note: For me, this situation breaks a lot, e.g. if a guest arrives around the same time as the repair guy. I found it best to save before calling the repair service.
Tom Peeping - If your sim is woohooing alone (masturbating) and notices him jerking off, they may
invite him to join.
*Note: I've had this situation fail as Tom thought he hadn't been invited in yet and couldn't find a path to the bed. Consider masturbating outside if you want Tom to join.