Why? To do so is to conflate liability with reality. The law does not determine whether something occurred. It doesn't determine whether your house was robbed or your car stolen. It doesn't determine whether the perpetrator committed a crime, whether the defendant robbed or stole. Instead, it determines whether a particular defendant is to be held criminally or civilly liable for the crime. A jury’s determination that the defendant is not guilty, does not magically restore your xbox to your entertainment center or your car to the driveway.
The ethical discussions of rape typically revolve around a model of consent.* Rape occurs when one person engages in a sexual act without the affirmative consent of another participant. Thus, a person should not engage in a sexual act with another without obtaining affirmative consent. In my view this model contains two key premises. First, a person should not act to harm another person. Second, that other people have the agency to determine what causes harm to themselves. Since people are not omniscient or psychic, the best way we can know another’s subjective thoughts is to inquire and trust that person’s assertions. Thus, to prevent harm a person must seek and rely on consent.
Notice this consent model is neutral as to the act involved. You shouldn't punch someone in the nose, perform a tonsillectomy on another person or insert an object in to another person's vagina, mouth, or rectum without their consent.**
Some feminists use a model of enthusiastic consent. There are varying definitions of enthusiastic consent, but essentially the idea is that you should only engage in sex activity with someone who has a sexual desire to engage in that activity with you. Under this model, an ethical person would seek to understand the sexual desires of the other person, and seek to fulfill those sexual desires rather than merely relying on a perhaps reluctant “yes.”
While I understand the rationale for enthusiastic consent, the model may deny the agency of people who are asexual, some sex workers, and others. As a result, I prefer a simple affirmative consent model. Essentially, a person must affirmatively consent to sexual acts. Negative consent, i.e. not saying no, is inadequate.