What should be done if a man has sex with a betrothed virgin in the city?

Stone them both to death.

If a damsel that is a virgin be betrothed unto an husband, and a man find her in the city, and lie with her;

Then ye shall bring them both out unto the gate of that city, and ye shall stone them with stones that they die;

the damsel, because she cried not, being in the city; and the man, because he hath humbled his neighbour's wife:

so thou shalt put away evil from among you. Deuteronomy 22:23-24