I'll just dare to add a few lines to the above answer. From my reading of Pride and Prejudice, what I have observed that the matrimonial theme is portrayed in such a way that, while being serious, the narrator is satirical many a times. Four marriages took place in the novel: between Collins and Charlotte, Lydia and Wickham, Elizabeth and Darcy, Jane and Bingley. Among these the first two were definitely used as instruments of satire. The latter two are ideal couples.
Charlotte married Collins for the sake of money, since in a nineteenth century English society especially where the property of the father was going to be entailed, money was a big factor. And, unwed aged girls were almost unacceptable in that context. So, in order to secure her fortune, Charlotte decided to marry Mr. Collins thinking very meticulously. But, while doing so, she did not bother a bit about her self-respect or love toward the person whom she was going to marry. On the contrary, Jane and Bingley loved each other, and finally, they turned their love-affair into marriage. Jane preferred love because she had chances to woo Darcy who was richer than Bingley. In their conjugal life, love would be dominant. But in case of the earlier couple, their would be no love in the true sense, rather Charlotte, after her marriage, sacrificing her dignity and acting according to her husband's will solely, had to be a Mrs. Collins, nothing else.
What Charlotte did, can be justified with many reasons, but if a man like Bingley you get and your mind allows to love him and at the same time, get love in exchange, then why should you not marry him? Bingley is a nice match for Jane who could make her happy both with love and the fortune he possessed. While Collins-like man can make that girl happy whose sole concern is fortune.
Jane Austen's "Pride and Prejudice" raises important moral issues concerned with the central theme of the novel namely the institution of marriage and other important aspects associated with the institution of marriage.
Jane Austen belongs to the Romantic Age in English literature.
"Pride and Prejudice"(1813) like all of Jane Austen's novels reflects faithfully the socio-economic conditions of what historians term as 'Regency England'(1811-20).
Since women of this period had no right to ownership of property they were financially dependent on their husbands,and hence the urgency and anxiety throughout the novel for the ladies to get married to "young men of large fortune" (ch. 1). This is why Mrs.Bennet is very anxious that somehow Bingley must marry her daughter Jane.
Love and romance certainly play an important role in the relationship between Jane and Bingley. Both of them are romantically inclined towards one another. However, Bingley is under the control of his friend Darcy. It is Darcy who is both responsible for separating Bingley and Jane and then reuniting them. One of the important reasons Darcy gives for separating Bingley from Jane is that he felt that Jane did not reveal much warmth or affection for Bingley. He writes to Elizabeth in ch.35:
Your sister [Jane] I also watched. -- Her look and manners were open, cheerful, and engaging as ever, but without any symptom of peculiar regard, and I remained convinced from the evening's scrutiny, that though she received his attentions with pleasure, she did not invite them by any participation of sentiment. Ch.35
However, later he accepts that he was mistaken and reunites them both.
Charlotte in conversation with Elizabeth advises her that Jane should take the initiative and quickly make it clear to Bingley that she is interested in getting married to him:
In nine cases out of ten, a woman had better shew more affection than she feels. Bingley likes your sister undoubtedly; but he may never do more than like her, if she does not help him on.'' Ch.6.
Elizabeth disagrees, but the readers realize that Charlotte has been right after all, because Darcy separated Bingley and Jane as he wasn't convinced that Jane was really in love with Bingley. If only Elizabeth had accepted Charlotte's advice and if Jane had boldly taken the initiative Jane and Bingley would have got married straightaway.
Charlotte is entirely unromantic. All that she is interested in is financial security in marriage. She is not interested in getting married to anyone in particular. She tells Elizabeth that the only thing that she is interested in is a comfortable home:
I am not romantic, you know. I never was. I ask only a comfortable home; and considering Mr. Collins's character, connections, and situation in life, I am convinced that my chance of happiness with him is as fair as most people can boast on entering the marriage state.'' Ch.22
So as soon as she hears that Elizabeth has rejected Collins she wastes no time and makes it plain to him that she is ready to marry him. Collins proposes and Charlotte immediately accepts.
Jane and Bingley and Charlotte and Collins represent the problem of how forthcoming the girl should be in matters of love. Should she be reticent like Jane and bide her time or be bold like Charlotte and take the initiative and grab her man.
Mr.Bennet's estate is 'entailed' to Mr. Collins because Mr.Bennet does not have a son. In 'Regency England' only male heirs could inherit the title and the estate of their fathers. The third paragraph of chapter 50 clearly reveals the 'economic' necessity of having a son and the disappointment at not being able to have one and the consequent predicament which Mr.Bennet faces in not being able to personally meet the financial demands of Wickham.
In Ch.33 Col.Fitzwilliam tells Elizabeth "I may suffer from the want of money. Younger sons cannot marry where they like." Clearly hinting at her impoverished status.
The central theme of the novel--how much money is necessary for a successful and a happy marriage--is explicitly stated by Elizabeth in in Ch.27 : "Pray, my dear aunt, what is the difference in matrimonial affairs, between the mercenary and the prudent motive? WHERE DOES DISCRETION END, AND AVARICE BEGIN?"
Was Col. Fitzwilliam Darcy 'discreet' or 'avaricious'?
The contrasting lifestyles of different social groups is structurally central to a Jane Austen novel. In "Pride and Prejudice" the landed gentry represented by Darcy is contrasted with the newly rich trading class represented by Bingley and his sisters.
The novel was written against the background of the threat of an invasion by Napoleon. The militia was a temporary voluntary force raised especially during times of a national emergency. Wickham was a member of this militia. Col.Fitzwilliam Darcy the younger son of an earl, on the contrary, is a fully commissioned officer of the regular army. In those days only an aristocrat or a member of the gentry could afford to purchase a commission in the army. In "Pride and Prejudice" Darcy purchases a commission for Wickham so that Wickham agrees to marry Lydia.
But most importantly the harsh reality of a bleak future for a dependent unwed old woman is hinted at when Charlotte Lucas' brothers are relieved that Collins is going to marry their sister, for otherwise they would have to look after her in her old age.