Matthew 5:27

We have here an exposition of the seventh commandment, given us by the same hand that made the law, and therefore was fittest to be the interpreter of it: it is the law against uncleanness, which fitly follows upon the former; that laid a restraint upon sinful passions, this upon sinful appetites, both which ought always to be under the government of reason and conscience, and if indulged, are equally pernicious. I. The command is here laid down (Mat 5:27), Thou shalt not commit adultery; which includes a prohibition of all other acts of uncleanness, and the desire of them: but the Pharisees, in their expositions of this command, made it to extend no further than the act of adultery, suggesting, that if the iniquity was only regarded in the heart, and went no further, God could not hear it, would not regard it (Psa 66:18), and therefore they thought it enough to be able to say that they were no adulterers, Luk 18:11. II. It is here explained in the strictness of it, in three things, which would seem new and strange to those who had been always governed by the tradition of the elders, and took all for oracular that they taught. 1.

— Matthew Henry's Concise Commentary (public domain)

Ye have heard that it was said--The words "by," or "to them of old time," in this verse are insufficiently supported, and probably were not in the original text. Thou shall not commit adultery--Interpreting this seventh, as they did the sixth commandment, the traditional perverters of the law restricted the breach of it to acts of criminal intercourse between, or with, married persons exclusively. Our Lord now dissipates such delusions.

— Jamieson-Fausset-Brown Commentary (public domain)