> The letter of the law allows anyone to be guilty of rape, in the U.S. at least

That actually varies based on jurisdiction, some states have separate laws with different offense names (often with similar conditions and punishments) for different kinds of sexual assaults, and still have “rape” being specific in the same way that it is in Britain, in some states and the federal system, “rape” is not a specific offense named in statute but is sometimes used as an informal name for some offenses. (E.g., in the federal system, the title of the relevant section including, but not limited to, the common law definition of “rape” is “sexual abuse” [0], and the word “rape” does not appear anywhere in it.)

[0] https://www.law.cornell.edu/uscode/text/18/2242

As an example, Trump was found civilly liable for Sexual Assault but could not be found liable for Rape because the jury found he only penetrated the victim with his fingers and not his penis.