If a man lies with his daughter-in-law, both must surely be put to death. They have acted perversely; their blood is upon them.
Sins against the seventh commandment are here ordered to be severely punished. These are sins which, of all others, fools are most apt to make a mock at; but God would teach those the heinousness of the guilt by the extremity of the punishment that would not otherwise be taught it. I. Lying with another man's wife was made a capital crime.
Commenting on Leviticus 20:10-21
If a man lie with his daughter in law,.... His son's wife, whether in the lifetime of his son, or after his death, or whether she was espoused or married, according to the above tradition (f): both of them shall surely be put to death; it being a breach of the law in Lev 18:15; and this is the penalty annexed to it, even death...
They have wrought confusion - See Leviticus 18, and especially the note on Lev 18:6 (note).