"Fornication" is unequivocally stated to be the only legitimate ground for which a man may put away his wife. The word used here is the more generic term for sexual uncleanness, namely fornication (porneia). This term may be used of all kinds of illicit sexual intercourse and may apply to such on the part of unmarried personas, in whose case the sin would not be in the specific sense of adultery. But though it is the generic word that is used here (cf. also Matt. 19:9), it is not to be supposed that the sense is perplexed thereby. What Jesus sets in the forefront is the sin of illicit sexual intercourse. It is, of course, implied that such on the part of a married woman is not only fornication but also adultery in the specific sense, for the simple reason that it constitutes sexual infidelity to her spouse. And this is the only case in which, according to Christ's unambiguous assertion, a man may dismiss his wife without being involved in the sin which Jesus proceeds to characterize as making his wife to be an adulteress.
John Murray