Jane Austen is satirizing the institution of marriage as it was considered in her day. In 1800 England, many women married, not for emotional or romantic intentions, but for social and economic security, and this caused a bit of dissatisfaction. Austen is clearly critical of a society where it is nearly impossible for women to be financially independent, and who are therefore forced into marriages for convenience. This is most clearly evidenced by the marriage of Charlotte Lucas to Mr. Collins. Charlotte very early in the novel tells Elizabeth that love in a marriage isn't important to her, so her marrying the rather annoying Mr. Collins doesn't come as that big of a surprise. If Charlotte had not married, she would have been destitute, so she makes the only choice she can towards self-preservation, and marries Collins. She makes the best of her situation and lives a comfortablee if not loving and joyful life.
The fact that Elizabeth is in a similarly dire situation but refuses Collins's proposal and states, emphatically, that she will only marry for love also emphasizes the satire. As the hero of the story, Austen makes it clear that this is the better attitude to have and this is the ideal that women should be allowed to live to. Elizabeth's strength in refusing not one but two unacceptable proposals is to be applauded, and we can then doubly enjoy the wonderful, romantic happy ending where Elizabeth gets it all -- the wealth, and more importantly, the love.
Pride and Prejudice is a satire on marriage because it presents five different couples with different reasons to marry, and the outcome of each couple is directly proportional to the reasons why they got married.
This being said, what Austen was trying to do was to show how much pressure society would place on women to attain a good marriage, because this was the only way they would make any gains in life in general.
Therefore, all the marriages that began for all the right reasons, ended up correctly. Those which began in chaos, ended the same way.
It is a satire because Elizabeth and Mr. Darcy were convinced for so long of their dislike for one another. At one point Elizabeth even says "From the very beginning, from the first moment I may almost say, of my aquaintance with you, your manners impressing me with the fullest belief of your arrogance, your conceit, and your selfish distain of the feelings of others, were such as to form that groundwork of disapprobation, on which succeeding events have built so immoveable a dislike; and I had not known you a month before I felt that you were the last man in the world whom I could ever be prevailed on to marry." And yet...they fall in love and get married. They both have such a complete and total change in opinion of one another, it's funny.