What do you mean by end the marriage? They technically have the right to end all sexual relations and to move out and live separately. They are strongly urged to avoid this and to instead forgive their spouse and work on their marriage, but they are not technically required to. Please don’t misunderstand me, I am strongly against a person refusing to forgive their unfaithful spouse, I fell very strongly that they ought to do everything they can to make their marriage work and should forgive their spouse to make this happen, but that doesn’t change the fact that they do, in fact, have a right to separate if their spouse is unfaithful to them. Refusing to tell them of ones infidelity prevents them from exercising this right and so should only occur when the spouse in question has made it clear that they would prefer to be kept in the dark about such infidelities. In such a case they have waived their right, in the case where a person hides their infidelity without such explicit permission from their spouse then they personally are preventing their spouse from having the opportunity to exercise their right if they so desired, and that is wrong. If the spouse does decide to separate (which I would always strongly advise against) the cause of that separation ultimately comes back to the offense of infidelity. That is one of the results, the temporal effects, if you will, of infidelity. Once the infidelity has occured one should not hide the affair simply out of a desire to prevent a separation as it is the affair itself they have already had which has severed the ties of marriage to the point of separation. That is like a kid breaking a vase and then hiding it from his parents so that he won’t be punished. And yes, I am well aware that it is not just the unfaithful spouse who will be punished, the children involved will also be punished, but again, that is the result of the affair, that is what infidelity does, that is part of why infidelity is such an unbelievably horrible thing. The more I think about all this the more convinced I am that unless a spouse has explicitly said they would not want to know they should be told.