Well, you wouldn't want to assume this tale represents Chaucer's position; he takes various positions that don't fit together smoothly in this set of stories. However, this story shows marriage as anything but the sacred bond the church would have it. Marriage is shown as entered into by the ignorant, who don't really know one another. This allows for deception, and makes those involved look stupid. Broadening the question a little, the story shows how sexual desire can make us look like fools.
I believe that "The Miller's Tale" is a cautionary tale about marriage that emphasizes the importance of equality and mutual respect between a husband and wife.
"The Miller's Tale" satirizes the stereotypical thought that a man should pursue and hopefully marry a younger woman. I'm not talking about a difference of a couple of years. I'm talking about a big age split. In the story, the carpenter marries a woman that is much younger than he is. He is married to a very attractive woman that is eighteen. Some men might think that marrying a younger "trophy wife" is awesome; however, the story warns against this kind of marriage. Because his wife is so young and so attractive, the carpenter lives in a constant state of jealousy. He is threatened by any man that looks at his wife, and this makes him incredibly protective of his wife. In fact, "wife" might be a generous term. Alisoun is basically a prize that he is trying desperately to hold on to. The marriage is not a healthy marriage between two equal partners.
The equality of marriage partners is another idea that "The Miller's Tale" is illustrating. The story illustrates how women are not typically given equal footing within a marriage. This is true of the marriage between the carpenter and his wife. He treats her like a possession instead of a partner. She exists for his sexual pleasure, and she resents being treated as an object. Objects are acted upon by subjects. Alisoun doesn't want to always be an object. She wants some control of a relationship, which is why she begins cheating on her husband. It's her way of taking control of a relationship and getting revenge on her husband.
It seems to me that the story is putting a big emphasis on the importance of marrying somebody of similar age. Similar ages will likely put two people at similar maturity levels. That should lead to increased respect between two people. That mutual respect will likely lead to a marriage with more equality between the husband and wife. I believe that Chaucer's main point of this story is to show how equality between partners is key to a healthy marriage.