The UK’s law is precedent based. The definition of rape thus goes back all the way to the 1800s (like many other restrictive laws that need to be revisited, e.g. classifying any transportation device with any kind of engine, i.e. not human or animal propelled, as a vehicle thus forcing the owners of e.g. low end e-scooters to have licences, registration, insurance etc. without providing the framework for any of these), wherein rape was almost exclusively committed by men, therefore lawmakers found it proper to define it as penetration of the victim using one’s genitalia - in a way to differentiate from “lesser” sexual assaults like flashing someone or forcing their hands on said genitalia.