27 "You have heard that it was said, 'Do not commit adultery.' 28 But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart. 29 If your right eye causes you to sin, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell. 30 And if your right hand causes you to sin, cut it off and throw it away. It is better for you to lose one part of your body than for your whole body to go into hell.
And here is the excerpt from Matthew Henry's Complete Commentary (with my highlights added in):
"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 (v. 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 (Ps. 66:18 ), and therefore they thought it enough to be able to say that they were no adulterers, Lu. 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. We are here taught, that there is such a thing as heart-adultery, adulterous thoughts and dispositions, which never proceed to the act of adultery or fornication; and perhaps the defilement which these give to the soul, that is here so clearly asserted, was not only included in the seventh commandment, but was signified and intended in many of those ceremonial pollutions under the law, for which they were to wash their clothes, and bathe their flesh inwater. Whosoever looketh on a woman (not only another man’s wife, as some would have it, but any woman), to lust after her, has committed adultery with her in his heart, v. 28. This command forbids not only the acts of fornication and adultery, but, (1.) All appetites to them, all lusting after the forbidden object; this is the beginning of the sin, lust conceiving (James. 1:15 ); it is a bad step towards the sin; and where the lust is dwelt upon and approved, and the wanton desire is rolled under the tongue as a sweet morsel, it is the commission of sin, as far as the heart can do it; there wants nothing but convenient opportunity for the sin itself. Adultera mens est—The mind is debauched.Ovid. Lust is conscience baffled or biassed: biassed, if it say nothing against the sin; baffled, if it prevail not in what is says. (2.) All approaches toward them; feeding the eye with the sight of the forbidden fruit; not only looking for that end, that I may lust; but looking till I do lust, or looking to gratify the lust, where further satisfaction cannot be obtained. The eye is both the inlet and outlet of a great deal of wickedness of this kind, witness Joseph’s mistress (Gen. 39:7 ), Samson (Jdg. 16:1 ), David, 2 Sa. 11:2 . We read the eyes full of adultery, that cannot cease from sin, 2 Pt. 2:14 . What need have we, therefore, with holy Job, to make a covenant with our eyes, to make this bargain with them that they should have the pleasure of beholding the light of the sun and the works of God, provided they would never fasten or dwell upon any thing that might occasion impure imaginations or desires; and under this penalty, that if they did, they must smart for it in penitential tears! Job. 31:1 . What have we the covering of the eyes for, but to restrain corrupt glances, and to keep out of their defiling impressions? This forbids also the using of any other of our senses to stir up lust. If ensnaring looks are forbidden fruit, much more unclean discourses, and wanton dalliances, the fuel and bellows of this hellish fire. These precepts are hedges about the law of heart-purity, v. 8. And if looking be lust, they who dress and deck, and expose themselves, with design to be looked at and lusted after (like Jezebel, that painted her face and tired her head, and looked out at the window ) are no less guilty. Men sin, but devils tempt to sin.2. That such looks and such dalliances are so very dangerous and destructive to the soul, that it is better to lose the eye and the hand that thus offend then to give way to the sin, and perish eternally in it. This lesson is here taught us, v. 29, v. 30. Corrupt nature would soon object against the prohibition of heart-adultery, that it is impossible to governed by it; "It is a hard saying, who can bear it? Flesh and blood cannot but look with pleasure upon a beautiful woman; and it is impossible to forbear lusting after and dallying with such an object.’’ Such pretences as these will scarcely be overcome by reason, and therefore must be argued against with the terrors of the Lord, and so they are here argued against.(1.) It is a severe operation that is here prescribed for the preventing of these fleshly lusts. If thy right eye offend thee, or cause thee to offend, by wanton glances, or wanton gazings, upon forbidden objects; if thy right hand offend thee, or cause thee to offend, by wanton dalliances; and if it were indeed impossible, as is pretended, to govern the eye and the hand, and they have been so accustomed to these wicked practices, that they will not be withheld from them; if there be no other way to restrain them (which, blessed be God, through his grace, there is), it were better for us to pluck out the eye, and cut off the hand, though the right eye,and right hand, the more honourable and useful, than to indulge them in sin to the ruin of the soul. And if this must be submitted to, at the thought of which nature startles, much more must we resolve to keep under the body, and to bring it into subjection; to live a life of mortification and self-denial; to keep a constant watch over our own hearts, and to suppress the first rising of lust and corruption there; to avoid the occasions of sin, to resist the beginnings of it, and to decline the company of those who will be a snare to us, though ever so pleasing; to keep out of harm’s way, and abridge ourselves in the use of lawful things, when we find them temptations to us; and to seek unto God for his grace, and depend upon that grace daily, and so to walk in the Spirit, as that we may not fulfil the lusts of the flesh; and this will be as effectual as cutting off a right hand or pulling out a right eye; and perhaps as much against the grain to flesh and blood; it is the destruction of the old man.(2.) It is a startling argument that is made use of to enforce this prescription (v. 29), and it is repeated in the same words (v. 30), because we are loth to hear such rough things; Isa. 30:10 . It is profitable for thee that one of thy members should perish, though it be an eye or a hand, which can be worse spared, and not that thy whole body should be cast into hell. Note, [1.] It is not unbecoming a minister of the gospel to preach of hell and damnation; nay, he must do it, for Christ himself did it; and we are unfaithful to our trust, if we give not warning of the wrath to come. [2.] There ar some sins from which we need to besaved with fear, particularly fleshly lusts, which are such natural brute beasts as cannot be checked, but by being frightened; cannot be kept from a forbidden tree, but by cherubim, with a flaming sword.[3.] When we are tempted to think it hard to deny ourselves, and to crucify fleshly lusts, we ought to consider how much harder it will be to lie for ever in the lake that burns with fire and brimstone; those do not know or do not believe what hell is, that will rather venture their eternal ruin in those flames, than deny themselves the gratification of a base and brutish lust. [4.] In hell there will be torments for the body; the whole body will be cast into hell, and there will be torment in every part of it; so that if we have a care of our own bodies, we shall possess them in sanctification and honour, and not in the lusts of uncleanness. [5.] Even those duties that are most unpleasant to flesh and blood, are profitable for us; and our Master requires nothing from us but what he knows to be for our advantage."