Rape is legal in Equestria, if:
>the rapist is in her estrus
>the stallion is not married, whether to a single mare or a herd
Often there are incidents when the raped male goes to the police station to try and sue the rapist, he gets double-teamed by both the policemare and his rapist. This most commonly happens when both are in heat. It can be hard for a single mare to control herself during her heat.
A rapist can be free from criminal liability if the herd decides that she's attractive enough.