"Certain BDSM sexual practices can be and sometimes are prosecuted under state criminal laws dealing with assault, aggravated assault, sexual assault or sexual abuse. This is true even when there is clear consent. Criminal prosecution can arise in various circumstances, including situations in which one participant is injured requiring medical treatment and the injury is brought to the attention of the police by hospital staff, doctors or a family member. In other cases, the BDSM encounter is more extreme than one participant expected, and the injured person contacts the police. Sometimes a person may feel guilt or shame for their participation in the experience after the fact. Or the relationship may have soured and they want to get revenge against the other person. Conversely, police may raid a BDSM event and witness conduct that they construe as unlawful. In other scenarios, video or pictures of BDSM activity may be turned into police, or discovered online by law enforcement officials.
... Technically speaking, any type of “impact play” is illegal. One person cannot legally give their consent to being physically assaulted. As far as the law and courts are concerned the issue becomes one of violence, not sex. The criminal offense is that one person is causing another person physical harm. ...
In a rape case, the sex act is not considered criminal unless it can be shown that one party did not consent. In a BDSM case, however, the causing of physical harm is in and of itself criminal. The question then becomes the extent to which the courts will allow such criminal conduct to be excused by one person’s consent.
The definitions of assault, abuse and other such crimes involving infliction of physical harm vary from state to state. There are no federal [statutes] in this area. Most state laws covering assault do not cite consent as a defense."https://pridelegal.com/assault-consent-bdsm-law/
(Mar 4, 2014)