Gǔdài duì sītōng xíngwéi yǒu yánlì de fǎlǜ chéngfá.
In ancient times, there were severe legal punishments for adultery.
Tips
usage
私通 covers two distinct meanings: (1) secret communication with the enemy (treason context), and (2) an illicit romantic/sexual relationship. Context always clarifies which is meant.