Jesus, via Matthew has a higher standard for his followers. Matthew has this as a solemn statement of Jesus (ἐγὼ δὲ λέγω ὑμῖν). Not only that, he repeated the same statement later in chapter 19:3-9, based on Mark, chapter 10:2-12. Luke also had this same statement in chapter 16:18. There was only one ground for divorce, the unchaste sexual immoral women (ὅτι πᾶς ὁ ἀπολύων τὴν γυναῖκα αὐτοῦ παρεκτὸς λόγου πορνείας). This immorality, “πορνείας” included all kinds of sexual perversity, not just adultery. Otherwise, that divorced women would become an adulteress, committing adultery (ποιεῖ αὐτὴν μοιχευθῆναι). If anyone married a divorced woman, they were also committing adultery (καὶ ὃς ἐὰν ἀπολελυμένην γαμήσῃ μοιχᾶται.). Thus, the reasons for divorce were explicitly limited to sexual perversity. However, there was no indication that a woman could divorce her husband for any sexual cruelty or perversity.
Once again, Matthew alone mentions one of the 10 Commandments from Exodus, chapter 20:14. This was something all the Jewish people knew, since they had heard it said often (Ἠκούσατε ὅτι ἐρρέθη). Everyone knew that they should not commit adultery (Οὐ μοιχεύσεις), having sex with a married woman who was not your wife. However, Matthew has this solemn statement of Jesus (γὼ δὲ λέγω ὑμῖν) that he was demanding more. Everyone who looked at a woman lustfully (ὅτι πᾶς ὁ βλέπων γυναῖκα πρὸς τὸ ἐπιθυμῆσαι αὐτὴν), has already committed adultery with her in his heart (ἤδη ἐμοίχευσεν αὐτὴν ἐν τῇ καρδίᾳ αὐτοῦ). This was a much higher standard, not merely the act of adultery, but the planning to do so was wrong. This was adultery of the heart.