This raises an interesting question of cultural background. It is not clear that a wife actually could officially divorce her husband in Palestine at the time of Jesus.  For example, the Jewish historian Josephus says that it was against the Jewish Law for a woman to divorce her husband, although it was possible for elite women to do it under Roman law.  The later Jewish collection of oral tradition, the Mishnah (ca. 200), similarly assumes throughout that only a husband could initiate a divorce. 
What then are we do with Mark 10:12, where Jesus prohibits a wife from divorcing her husband? It is a difficult question. On the one hand, the time of Jesus has left us such little evidence that we cannot ignore the possibility that, despite all the information we have from elsewhere, it was still possible somehow in Galilee for an ordinary wife to initiate a divorce. Or perhaps Mark is worded imprecisely and refers to a wife asking her husband for a divorce.
However, the more likely suggestion is that the oral tradition of Jesus' statement on divorce expanded as the saying left Palestine. In the Roman world, it was equally possible for wife to divorce her husband as for a husband to divorce a wife. Mark or the tradition he inherited thus would have taken an original saying, which only mentioned the husband divorcing, and expanded it for a Roman audience to include the wife as well.
The original concern of Jesus in Galilee, then, would have focused exclusively on husbands throwing away their helpless wives. This likelihood changes our sense of the original dynamic in the ministry of Jesus considerably. Jesus' teaching, as we would expect, focused on compassion toward the powerless, namely, the wife in Galilean society. Mark 10 appropriately broadens the principle for a different context.
Another curious aspect of Jesus' teaching here, and perhaps quite revolutionary, is the sense that a husband can commit adultery against his wife (Mark 10:11). The New Testament anthropologist Bruce Malina has argued that adultery at the time by definition was the shaming of a man by sleeping with his wife.  Accordingly, a man could not commit adultery against a woman. If a man slept with another man's wife, he was committing adultery against her husband. Similarly, if a married man slept with a single woman, he would not be committing adultery.
If Malina's analysis of ancient Mediterranean culture is correct, then it would have been shocking for Jesus to suggest that a man might commit adultery against his wife by sleeping with another woman. Jesus would be giving a significance to the wife that was unheard of. What we assume when we read these texts--that adultery goes both ways--becomes another example of Jesus' startling care for the disempowered in society and his shocking preaching.
Matthew 5:32 then must also be somewhat tongue in cheek, typical Jesus. A divorced wife will almost have to remarry to support herself. In that sense, the husband who divorces her is forcing her to sleep with another man. Tongue in cheek, he is forcing her to commit adultery against himself. Obviously Jesus could not mean this literally. He is shaming the man who would do such a thing. The man who divorces his wife, in effect, is forcing her to shame him by sleeping with another man.
Again, we see that these statements by Jesus are far from legalistic rules...
 A recent article discussing the issue is Robert Brody, "Evidence for Divorce by Jewish Women?" Journal of Jewish Studies 50.2 (1999): **
 Antiquities 15.259.
 Yev 14.1; Qid 1.1; Git 4.2, 8.4, 9.10
 The Social World of Jesus and the Gospels.