What? know you not that he who is joined to a harlot is one body? for two, says he, shall be one flesh.
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Paul says this because the person who involves himself in contamination is united with the person with whom he involves himself. Sexual immorality makes them both one, in nature as well as in sin. Commentary on Paul’s Epistles.
Know ye not that he which is joined to an harlot is one body? One body by a union and blending of the two bodies. Just as merchants in partnership have but one capital, because it is common to both, so those who join in committing fornication have one body, because their bodies are common to both, as Cajetan says. So two are one flesh: that Isaiah , out of two there is made but one human being, and that not spiritual, but carnal—wholly fleshly.
For two, saith Hebrews , shall be one flesh. S. Paul is here quoting from Gen. ii24 , where the words are applied to those married. But he refers them truly enough to fornicators, because the external Acts , whether of them or of those married, do not differ in kind, though they differ morally by the whole sky, for the acts of the former are lustful and vicious, but those of the latter are acts of temperance, righteousness, and virtue, as S. Thomas says.
1. Observe that it is said of the married that they too shall be one flesh (1.) by carnal ...