Do the members of a married couple with children have a moral obligation, not (just) to each other, but to their children, to not cheat on each other?
I'm guessing what the questioner is wondering is roughly this-- "Considering the way that infidelity tends to increase the probability of divorce and considering the known ill effects of divorce on children, do couples have a duty not just to each other, but also to their children, to be faithful?" My answer to that question is: yes. Then there's the question whether people should be faithful even if they've redefined marriage so that infidelity is truly a non-issue (if such people exist). And there's also the question whether we have a duty to children not to redefine marriage that way. No, and no, to those two questions. But if the question is about people in ordinary marriages, I'll go for the idea that infidelity puts children in jeopardy, so the obligation to refrain is partly to one's children.