Mariama Bâ's portrait of marriage is definitely a complicated one. This might get you started!
Both Ramatoulaye and Aissatou have happy, successful marriages at first; they choose among a variety of suitors and both can and do marry for love despite external pressures (Ramatoulaye's mother disapproves of her choice and thinks she could do better than a lawyer from a family with social status beneath their own, while Aissatou's in-laws think she's not quite good enough for the doctor son of a tribal princess). In this way we see that in embarking on a marriage, women of Ramatoulaye and Aissatou's generation face challenging social and cultural pressures—but that they are able to overcome them to make decisions they are happy with.
Unfortunately, their marriages are eventually threatened by the same cultural pressures. Aissatou's husband Mawdo unhappily remarries at the insistence of his mother, who has handpicked his second bride and threatens to estrange herself from him if he doesn't accept her choice. He accepts the marriage, but intends to stay with Aissatou, the wife he loves, as much as is possible. Aissatou, however, refuses to be part of the polygamous tradition she regards as insulting and divorces Mawdo, taking his four sons with her. Ramatoulaye's husband also takes a second wife; smitten with the very young and very beautiful Binetou, he abandons Ramatoulaye and his twelve sons. Ramatoulaye, however, decides not to divorce him and remains with him until his death, as does Binetou, who has little affection for him but, at her mother's insistence, marries for his money and influence. Ramatoulaye turns down other suitors after her husband's death; she refuses to marry for anything less than love (esteem is not enough). Thus we see that marriage for the older generation is deeply bound up in cultural pressures, assumptions, and controversies (is polygamy okay or not okay?), but that even in the most difficult situations both women and men have options and the agency to choose between them. (That said, to some extent women especially are exposed to the vicissitudes of fate; their husbands can make decisions that the women have little to no control over. They can only control how they themselves choose to act in response.)
It seems that the younger generation has a greater degree of flexibility in marriage; Ramatoulaye's children lead different lives. Daba, the eldest, is happily married to a partner who treats her like an equal. Three of her daughters smoke and wear pants (not dresses). And a fourth is pregnant out of wedlock—but is not shunned by her family or the family of her child's father. She is still accepted and loved, taken care of until she can graduate and marry the man she loves.
Thus it seems that attitudes toward marriage are changing; though women have always had choices, the cultural and social pressures surrounding marriage often led (and still do sometimes lead, as with Binetou) to disaster. Equal marriages and equality between the sexes, as well as tolerance for those who don't follow the norms, appear to be growing over time. As the novel closes the future is a source of hope—something to look forward to.