While many a reader has wished Mary Crawford to marry Edmund and Fanny to marry Henry, the ending is not forced. Rather, the characters receive their (mostly) just desserts based on character flaws or strengths that have long been established. Ultimately, neither Mary nor Henry take seriously the pain a lack of moral values can cause other people. Henry falls victim to his pride and must regain Maria's affections, even at the risk of losing Fanny. He never seems to care that having an affair might wreck Maria's marriage or her life, as it does. Likewise, Mary finds it impossible to take Henry's affair with Maria seriously. It was what people in her social set generally did: her main concern was to cover it up.
Through running off with Maria, Henry destroys the one relationship that offered him any hope of redemption. Through her inability to register any distress over Henry and Maria, Mary alienates Edmund, the person she had hoped to marry despite his, to her mind, lack of ambition in becoming a clergyman. Both have to live with the consequences of what they do and say. Maria is sent into exile with Mrs. Norris once her marriage breaks up: Jane Austen does note that Maria gets punished more harshly than Henry, and unfairly so, because she is a woman.
Fanny, who has suffered "the pains of tyranny, ridicule, and neglect" as the poor relation in the Bertram household, has learned self-discipline and the importance of treating people with respect. Likewise, Edmund, as the second son, has had his needs subordinated to those of his older brother and has learned to be considerate and kind. Each is worthy of the other, and as Jane Austen puts it, tongue-in-cheek, she, indeed, will not force anything:
I purposely abstain dates on this occasion, that every one may be at liberty to fix their own ... I only entreat everybody to believe that exactly at the time when it was quite natural ... Edmund became as anxious to marry Fanny as Fanny herself could desire.