[Ovid]

The best way for him to preserve the family unit is to zip his lip. Dad is most likely content with getting quiet pussy on the side, and is disinclined to destroy his family, social position, and wallet by divorcing mom. On the other hand, if mom gets the merest hint about what's going on, she'll likely make dad's life miserable by keeping tabs on him constantly (making him more likely to contemplate divorce) or she'll stomp her foot and demand a divorce herself. And then the family will be pretty much irretrievably buggered.