John Nolland points out in his commentary on Matthew that the combination “evil and adulterous” is found in the LXX of Hosea 3:1, applied to Gomer. He suggests that by using this phrase, Jesus is echoing Hosea, and implicitly comparing the Jews to the generation of the exile.

This makes sense of the following verses in Matthew 12, where Jesus moves from condemning the adulterous generation to saying that He will give the sign of Jonah: And Jonah, of course, was the prophet who delivered Nineveh from destruction, so that the Assyrians survived to destroy the Northern kingdom. Hosea’s “evil and adulterous” bride/generation was, in fact, the same generation as Jonah’s (both during the days of Jeroboam II, Hosea 1:1; 2 Kings 14:23-25).

This reference helps to specify the nature of the adultery of the scribes and Pharisees. Hosea 3:1 explicitly compares the adulteress to Israel: “Go again, love a woman who is loved by her husband, yet an adulteress, even as Yahweh loves the sons of Israel, though they turn to other gods and love raisin cakes.” By asking for a sign, Jesus implies, the scribes and Pharisees are betraying their love for other gods.

The logic, presumably, is that Jesus is the bridegroom (9:15), but that the demand for a sign veils the bride’s preference for another. And, of course, is this generation is “adulterous,” then the bride/generation should be expecting to receive divorce papers.