In order to really understand the major themes of this novel, you must consider how each of the women in the novel contribute to the reader's understanding of importance of marriage and the motivations for marriage. The most significant characters to consider, if you must limit yourself, are Elizabeth, Charlotte, and Lydia. When you evaluate these three, you will see that one marries for love, one for financial security, and one to save her reputation after some inappropriate lusty behavior.
Starting with the silliest of the characters, it is made clear from the very start that Lydia is a flirt to the highest degree, boy-crazy, and all-in-all just a foolish girl. She doesn't think of the consequences of her actions and is stupidly seduced into "eloping" with Wickham, but eloping doesn't have the definition we think of today. Wickham has no intention of actually marrying a girl like Lydia who has no fortune -- he ran away with her to "have his way with her." It is only when the family discovers what has happened and Darcy steps in with very large financial inducement that Wickham actually agrees to make the relationship legitimate and marry Lydia. Darcy steps in her to save Lydia and the rest of Bennet's reputations. Lydia, initially, seems to like the idea of being the first daughter married, but we see that pretty quickly, the glow is off the relationship and they need to struggle in their current financial situation.
The novel really makes its point about marriage in its presentation of Charlotte and Elizabeth. Very early in the novel Elizabeth declares that she would never consider marrying for anything less than true love. Charlotte though is more practical. She claims that love isn't a factor for her and that her most important concern is to marry someone who can provide a sustainable financial security to her. She is fully aware of how dire her situation will be if her parents die and leave her destitute. While Elizabeth rejects Mr. Collins proposal of marriage because she cannot imagine marrying someone she so loathes, Charlotte accepts his proposal because she sees in him a man who has a position in the church, a benefactor of Lady Catherine, and who will eventually inherit the Bennet estate. While he may be annoying (she sends out to garden quite a bit) she is content because she is cared for.
Elizabeth shows the true courage of her convictions most evidently when she turns down Darcy's first proposal. She has absolutely no confidence in his expressions of his feelings for her and he doesn't do a great job of conveying them in the first place. While she could have jumped at the proposal and lived a grand life, she once again asks herself if she could love the man, at in that moment, her answer to herself is no, so she rejects the proposal. It is only after several other events clarify for her the true nature and character of the man and his feelings for her that she changes her mind and happily accepts the second proposal. The end of the novel shows the triumph of her marriage true love when she soundly tells Lady Catherine that she will do as she pleases and will follow her heart, not merely the dictates or traditions of society. The novel ends with the clear prediction of their "happily ever after."