How is the Theme of Marriage Explored Throughout the Novel?

Tolstoy’s novel begins with one of the most famous openings in all of literature: “All happy families are alike; each unhappy family is unhappy in its own way.” This line signals the novel’s most pressing and consistent theme: marriage.
Hailed by some as the greatest work ever written on marriage, Anna Karenina examines many aspects of this social institution, particularly as it relates to individual passions, conventional expectations, religion and social status. The idea of marriage and the practice of marriage dominate Tolstoy’s narrative to such an extent the book achieves a single-mindedness of purpose, rarely relenting in its exploration of the ways that marriage influences the lives of the characters.
Importantly, marriage mediates more than the relationships between the individuals married to one another. Marriage also mediates the relationships that individuals have with the world at large. We see this pointedly in the character of Anna as she suffers the opprobrium of society after leaving her husband for Count Vronsky. Her former friends abandon her. Even Dolly, who loves Anna deeply, finds that her friendship with Anna is irreparably altered by Anna’s decisions regarding her marriage. Thus Anna's marriage status affects more than just her relationship to her husband. It affects her relationships with her social world as well.
The various marriages in the novel stand as differing vantage points for the many comments the novel makes on marriage and how this one social institution shapes the inner and outer lives of its characters.
Indeed, for the most part each marriage is unhappy in its own way. Dolly and Oblonsky face a challenge that is somewhat similar to that faced by Karenin and Anna, yet they choose to deal with their problems very differently. Where Dolly chooses dispassion, Anna chooses passion. Their decisions, in turn, determine their living situations, material situations, and levels of happiness.
Reading the novel as an exploration of the institution of marriage, a rather distinct context arises wherein the story-lines of each character can be understood as examples of how marriage can influence (positively and negatively) a person's life, livelihood and social standing. Subordinate to the topical theme of marriage, threads of inquiry and commentary run through the novel as to the generic nature of young love (Levin and Kitty), the turmoil of loveless marriage (Anna, Dolly, Karenin), and the role of love in child rearing.
While marriage is thoroughly parsed in the novel and shown to be far from the simple, blissful fulfillment of all life’s promises, Anna Karenina does not ultimately posit a statement on the virtues of the institution. Instead, Tolstoy’s novel offers a rich and complex view of marriage, suggesting by an accumulation of instances that marriage is a substantial social and personal force in society.