John Chrysostom

But let us see in what sense "Marriage is honorable in all and the bed undefiled." Because (he means) it preserves the believer in chastity. Here he also alludes to the Jews, because they accounted the woman after childbirth polluted: and "whosoever comes from the bed," it is said, "is not clean." Those things are not polluted which arise from nature O ungrateful and senseless Jew, but those which arise from choice. For if "marriage is honorable" and pure, why forsooth dost thou think that one is even polluted by it?

Homily on Hebrews 33 ↗

Theophylact of Ohrid

See how often he speaks about chastity. Above, when mentioning the holiness of marriage (Heb. 12:14), he spoke about this chastity, and again after this he speaks about adulterers and fornicators. Or: in all circumstances and at all times. Not only in affliction and not only in times of peace: not "honorable" in this respect only, and not in another, but let it be entirely honorable. Here also the heretics who slander marriage are put to shame. For behold, it is called honorable precisely because it is preserved in chastity. …

Commentary on Hebrews ↗

Oecumenius

"Marriage is honorable in all things." How much indeed is the discourse about moderation! For also earlier, recalling sanctification, Paul spoke about this, and again after this he will speak about prostitutes and adulterers. Therefore, in all things, that is, not honorable in this part and not in another, but entirely throughout honorable and unblemished, let it be. In all ways and at all times; not in distress, but in ease let it not be. …

The Pseudo-Oecumenian Catena on Hebrews ↗

Augustine of Hippo

Sometimes people who commit this sin treat it lightly out of heaven knows what kind of perversity. They hunt about for heaven knows what null and worthless proofs in their support, and say, "God does not mind the sins of the flesh." Well, what about what we have heard today, "God will judge the immoral and adulterous"? So there you are, pay attention, any of you afflicted with this sort of disease. …

Sermon 32.11 ↗

Thomas Aquinas

Then (v. 4) he urges them to do good to themselves. In regard to this he does two things: first, he gives an admonition against the concupiscences of carnal pleasures; secondly, against coveting external things (v. 5). It should be noted that sin in regard to sex occurs in two ways: in one way by the illicit union of one married person with another; as to this he says, Let marriage be held in honor among all who would not be continent: not fornication. …

Commentary on Hebrews ↗