How is the theme marriage expressed in the play Macbeth?
In Macbeth, there are two marriages of importance in the play: the Macbeths and the Macduffs. Both end badly. Three out of four spouses end up dead: Lady Macbeth (suicide); Lady Macduff (murder); Macbeth (beheading). Only Macduff survives.
It is important to note that the Macbeths have no kids, nor do they want any, while the Macduffs have a child, who dies with his mother. Children and the death of children are major motifs in the play, and they symbolize the byproducts of unnatural and sexist marriages that existed in the feudal system of Medieval Scotland.
Gender also plays a major role within the marriages. Women are on the bottom rungs of the socio-economic ladder within the feudal system and the Great Chain of Being. There isn't much difference between a witch and a Lady; in fact, the witches may have had more influence and mobility than Lady Macbeth or Macduff. Needless to say that both Lady Macbeth and Macduff are confined to the home because it is a man's world within and beyond the castle.
Overall, Shakespeare presents a sexist society in which both thane's marriages are segregated in terms of gender roles (the males go to work, while the females stay home). Ultimately, both thanes place more importance on power or country than they do on loyalty to their spouses.
The Macbeths' marriage, ironically, goes well when both plan Duncan's murder together. Before, we saw a division of labor: Macbeth out fighting, while Lady Macbeth waiting helplessly at home. After the murder, though, the marriage falls apart because of mental illness, primarily Lady Macbeth's paranoia, sleepwalking, and suicidal thoughts. In the Macbeths' marriage, Shakespeare shows the ultimate powerlessness of wives: Lady Macbeth can only attain power through her husband, and even then, it drives her to death. In other words, wives do not have the ruthlessness to kill and, thus, be a part of a thane's world.
In the Macduff marriage, Lady Macduff is furious that Macduff has left their castle defenseless. While Macduff is off in England, Macbeth has sent assassins to castle Macduff to kill his wife and child. This also is a division of labor, in that Macduff swears more allegiance to Malcolm and England than to his own wife and child. In the end, he feels guilt for the decision, but he nonetheless defeats Macbeth more for Scotland than for revenge against Macbeth's crimes.
In all honesty, marriage is not conveyed as the best thing. Lady Macbeth pushes Macbeth into committing regicide. She is his "evil side". Lady Macbeth talks Macbeth into killing the king in Act I. Their relationship starts with her "wearing-the-pants" in the relationship. Some may say it was an insecurity issue because in Act II, she is no longer controlling her husband, but they are more as equals. By Act III, she is more quiet and almost controlled by Macbeth. In Act V, she begins to feel ignored and her guilty consience gets the best of her because Macbeth is not there to tell her otherwise. She begins having fits of sleepwalking and then, off-stage, dies. When she dies, Macbeth states that he has forgotten the taste of fears. He then goes on to state that life is worthless and no one will be remembered. He also does not grieve the loss of his wife because Dunsinane is being marched upon.
Hope it helps!