As in, in some circumstances a particular marriage is compulsory, and then you have to make it even if you are already married. But you can’t take a second spouse unless you are forced to.
Example: June is a member of the Zumil masyarakat only by virtue of being married to Emil. If she leaves the masyarakat it will have to liquidate property or incur debt to return her share. Emil has a brother Ezra, who is married to Donna; they have a son Edhi. Ezra dies of pancreatic cancer. Donna is a member of the Zumil by birth, but she’s entitled to demand a levirate marriage anyway. She fancies Emil and doesn’t fancy becoming a widow, so she asserts her right. Emil has to marry her, and can because he must. Emil’s nephew Edhi thus becomes his stepson. A bit later Edhi falls in love with his maternal cross-cousin Dana, which would be a very suitable match. They plan to marry where Edhi turns eighteen. But then Emil is killed in a tractor-rollover accident. Edhi doesn’t have to marry his mother, because that would be incest; Donna becomes a widow, with attendant loss of status and conjugal rights. But Edhi isn’t related to his stepmother/aunt. June doesn’t want to be a penniless widow, and the masyarakat doesn’t want to pay her off. So she exercises her right to a levirate marriage. Emil’s heir-general is Edhi, so Edhi has to marry June. Edhi is only seventeen, too young to marry Dana, but he can marry June because he must. A bit later Edhi turns eighteen, but now he can’t marry Dana because that would be bigamy. Edhi and Dana conduct an illicit affair, which nobody minds so long as they are discreet, except for June, and June is not willing to seek her only remedy, divorce. She makes Edhi’s home life as miserable as she can manage.