Exlakewooder: Touching isn’t a Harchakah. It’s a Din D’oraisa. And whether a Niddah is an Ervah or not is a Machlokes between the Rambam and Tosfos. The Shulchan Aruch Paskens like the Rambam that it is Yeihareg V’al Ya’avor.
(In the example someone gave above the girl would not be required to be killed since that wouldn’t be Derech Chibah. Also, I think if the Shaila was ever asked L’ma’aseh that many, many Poskim would be willing to be Somech on Tosfos for the person to not allow themselves to be killed.)