Marital unfaithfulness is the ultimate act of selfishness. When one partner is unfaithful, this undermines one of the basic foundations of the marriage. Infidelity is always very painful for everyone, especially the faithful partner. It puts both partners at risk for sexually transmitted disease , one partner by choice and the other unknowingly. Usually there's a great deal of secrecy involved.
Infidelity can result in pregnancy involving an outside party.
The lies are compounded by a secret abortion, questionable paternity, or a child by another woman. This kind of trouble is never worth the few fleeting moments of enjoyment from the affair. The sooner the problem is brought to the light the better. The longer this charade continues the more you degrade yourself and your whole family. Demand that the problem be resolved immediately.