We happen to live in a culture where it's uncool to use rape metaphors. There's not necessarily a rhyme or reason to why any given language and time accepts one thing and not another though. To take a simple example, if you call me a dick or an ass, that's (mildly) offensive in a way that metaphorically calling me a nose or an arm would not be. Why? Because for whatever reason, we've come to associate those parts with insults, and not others*. Go back a few centuries (or to Québec today) and you'll find the most brutal swears are all religious terms that sound silly to most modern English speakers. The culture changes, and the language goes with it.

For all we know, rape metaphors will be perfectly fine in 150 years, but calling someone a carnivore or something will be the height of rudeness.

*Sure, there are specific reasons why this has likely developed in English in recent decades, revolving around issues of respect and awareness over how women have been historically viewed and treated, but I just meant that as a general matter these sorts of shifts aren't usually very predictable.