The protagonist or a love interest of the protagonist encourages their partner to cheat on him/her, either by asking their partner or by having a third party seduce or r*pe their partner. The intention behind it may range anywhere from a fetish to childmaking (when there's issues in that regard for the original couple). If the party left out is not supportive or if the involved party is unwilling at the start but comes to like the situation, use "Netorare". If there's no evolution from unwillingness to infatuation, use "Adultery" instead.