MATTHEW 19 Now it came to pass, when Jesus had finished these sayings, that He departed from Galilee and came to the region of Judea beyond the Jordan. 2 And great multitudes followed Him, and He healed them there.3 The Pharisees also came to Him, testing Him, and saying to Him, “Is it lawful for a man to divorce his wife for just any reason?”4 And He answered and said to them, “Have you not read that He who made them at the beginning ‘made them male and female,’ 5 and said, ‘For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh’? 6 So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate.”7 They said to Him, “Why then did Moses command to give a certificate of divorce, and to put her away?”8 He said to them, “Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so. 9 And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery.”10 His disciples said to Him, “If such is the case of the man with his wife, it is better not to marry.”

To commit adultery is to have relations with a woman who is not one’s proper spouse. A man commits adultery if he brings another woman in, instead of the one to whom he was lawfully bound. The law forbade obvious adultery, which is when another man seduces the woman in the house. But the Savior includes also that adultery that has not become known to everyone or that has not been proven as having occurred physically, because it is still adultery. Moreover, Christ agrees that the unfaithful wife has rebelled because she herself has destroyed the natural yoking and is no longer treated as a wife by her husband.
(Apoloinarius)