2,019. Starting Hilchos Issurei Biah

Hilchos Issurei Biah 1:1

If a person is voluntarily intimate with one of the relationships prohibited by the Torah, he is liable for the penalty of kareis (spiritual excision) as per Leviticus 18:29, “Anyone who performs any of these abominations, their souls will be cut off....” The plural “souls” includes both the man and the woman. If they violated the law unwittingly, they must bring a fixed sin offering. There are some prohibited relationships for which intimacy is punishable by execution in addition to kareis, which applies in all cases.

Hilchos Issurei Biah 1:2

When it comes to the prohibited relationships that are punishable by execution, if there were witnesses, they issued a warning, and the violators didn’t stop, then they’re executed through the prescribed form.