This world invisaged the institution of marriage differently due to most male infants being born dead. Marriages would occur between sister-wives, who would share a man between them. This form of marriage would take the form of a collection of woman-woman unions, with the male serving as a partner. These marriages between women wouldn't be sexual, but seen as a pragmatic solution that provides a stable way for them to have children and build lineages. It is common to marry a business partner to combine professional interests, or a political ally to build relationships and power blocks. All children produced within this group marriage would belong to that family, with lineage passed down through matrilineal lines.
These unions could extend to two or more women, but be limited to one husband to prevent conflict between families over reproductive rights. This makes patrilineal lines so important, because they are the only familial ties that connect all the siblings together when the women are not related. However, as there are so few men in society, it would make more practical sense to seek as many relationships as possible to secure a lineage. This would incentivize wives to seek lovers outside of their respected "vows". As there would be no way to determine who the real father is, this would be impossible to regulate.
In our world, when a spouse cheats on a partner, it is customary to leave the relationship. However these unions were not based on emotional commitments, but business relationships or to satisfy political realities based on power. as such, it would be a weak excuse to end it. Since these unions combine resources and assets to form a cohesive family, it would be difficult and costly to extract oneself from the relationship that you have invested in.
How can you discourage these kinds of relationships outside of group marriages?