>>Should extramarital sex be illegal? <<
What you do in your bedroom is between you and God. The government need not get into it.
But we need to turn the minds of people and make them WANT to wait. We’ve done a poor job of that.
Adultery is a tort. The deceived spouse, or the children should be able to bring a case in civil court, seek damages from the other party, and should a divorce result, make the adulterer suffer in the financial settlement and in custody decisions.
Extramarital activity of unmarried adults is their own business.
If you are a christian or Jew, then you would follow the Ten Commandments. "Thou shalt not covet thy neighbor's wife".
In some secular societies it is but it always has been with the ultimate Judge.
THou shalt not commit adultery is a good beginning.
We do not have to have a government tell us what to do.
God already told us and He gave us His commands to keep us happy, loving and fruitful.