I agree that it is useful, of not obligatory, to question mores that have traditionally been relgious in nature. The changing attitudes towards homosexuality, for instance, is one of the more fortunate results of the diminishing role of religion as the arbiter of moral standards.

However, sexual monogamy is not quite in the same boat as homosexuality, in the sense of the latter being something proscribed for arbitrary religious reasons. I think monogamy has as much to do with trust and honesty as with sex. The simple fact is, for most couples there is an expectation that their partner will not have other sexual partners. One reason for this might be that it is difficult to become sexually involved with someone without also becoming emotionally involved.

In any event, I think it is important to view such issues in terms of actual harm done to other individuals, even if that harm is strictly emotional, rather than in terms of adherence to abstract moral values. And certainly, too often, jealousy has prompted the supposedly "wronged" party to commit acts far more heinous than a mere affair.