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. The adulterer and the adulteress that had joined in the sin must fall alike under the sentence: they shall both be put to death, Lev 20:10. Long before this, even in Job's time, this was reputed a heinous crime and an iniquity to be punished by the judges, Job 31:11. It is a presumptuous contempt of an ordinance of God, and a violation of his covenant, Pro 2:17. It is an irreparable wrong to the injured husband, and debauches the mind and conscience of both the offenders as much as any thing. It is a sin which headstrong and unbridled lusts hurry men violently to, and therefore it needs such a powerful restraint as this. It is a sin which defiles a land and brings down God's judgments upon it, which disquiets families, and tends to the ruin of all virtue and religion, and therefore is fit to be animadverted upon by the conservators of the public peace: but see Joh 8:3-11.
II. Incestuous connections, whether by marriage or not. 1. Some of them were to be punished with death, as a man's lying with his father's wife, Lev 20:11. Reuben would have been put to death for his crime (Gen 35:22) if this law had been then made. It was the sin of the incestuous Corinthian, for which he was to be delivered unto Satan, Co1 5:1, Co1 5:5. A man's debauching his daughter-in-law, or his mother-in-law, or his sister, was likewise to be punished with death, Lev 20:12, Lev 20:14, Lev 20:17. 2. Others of them God would punish with the curse of barrenness, as a man's defiling his aunt, or his brother's wife (Lev 20:19-21): They shall die childless. Those that keep not within the divine rules of marriage forfeit the blessings of marriage: They shall commit whoredom, and shall not increase, Hos 4:10. Nay it is said, They shall bear their iniquity, that is, though they be not immediately cut off by the hand either of God or man for this sin, yet the guilt of it shall lie upon them, to be reckoned for another day, and not be purged with sacrifice or offering.
III. The unnatural lusts of sodomy and bestiality (sins not to be mentioned without horror) were to be punished with death, as they are at this day by our law, Lev 20:13, Lev 20:15, Lev 20:16. Even the beast that was thus abused was to be killed with the sinner, who was thereby openly put to the greater shame: and the villany was thus represented as in the highest degree execrable and abominable, all occasions of the remembrance or mention of it being to be taken away. Even the unseasonable use of the marriage, if presumptuous, and in contempt of the law, would expose the offenders to the just judgment of God: they shall be cut off, Lev 20:18. For this is the will of God, that every man should possess his vessel (and the wife is called the weaker vessel) in sanctification and honour, as becomes saints.

They have wrought confusion - See Leviticus 18, and especially the note on Lev 18:6 (note).

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: have been guilty of a shocking and shameful mixture, as Jarchi and Ben Gersom, as well as confounded the degrees of relation and affinity:

their blood shall be upon them; it being a capital crime, their, blood shall be shed for it; they shall be found guilty of death by stoning, as the Targum of Jonathan.

(f) Misn. ut supra. (Sanhedrin, c. 7. sect. 4.)

Sanctify yourselves therefore, and be ye holy--The minute specification of the incestuous and unnatural crimes here enumerated shows their sad prevalence amongst the idolatrous nations around, and the extreme proneness of the Israelites to follow the customs of their neighbors. It is to be understood, that, whenever mention is made that the offender was "to be put to death" without describing the mode, stoning is meant. The only instance of another form of capital punishment occurs in Lev 20:14, that of being burnt with fire; and yet it is probable that even here death was first inflicted by stoning, and the body of the criminal afterwards consumed by fire (Jos 7:15).