Prof Plum, yes I'm in the US. The moral outrage isn't for the breakdown of a marriage per se, nor is it particularly religion-motivated. It's for the person who feels it is 'ok' to cheat whether in a marriage or a relationship of any kind in which the partners have decided to be exclusive. Yes, marriage is a legal contract, and if a marriage/contract has been breached (in legal terms) then the injured party has a right to break that contract, obviously. And in legal terms, the cheater has 'broken' the contract by cheating. In the same way we would frown upon a businessperson's cheating their business partner, we should also frown upon a cheater, but instead we 'forgive' or 'ignore it'. Or worse, some actually think it makes the cheater a 'rico suave' (that may be a US only term, means 'Mr or Ms Smooth and Cool'). If your business partner had cheated you out of profits or 'double-dealed' you, would you forgive and just 'get along'? No, you would be royally pissed and would expect your professional circle to not deal with that person, not to mention the legal consequences. It doesn't have anything to do with religion itself, it has to do with promises made and broken. When two people stand in front of whoever (judge, registrar, clergy) and promise each other to be faithful, they each have a right to expect that promise to be kept, and a duty to keep that promise themselves. If they can't, then they either should commit, or should leave the relationship.
I guess what I'm trying to say is if you no longer love your partner, for whatever reason, the correct thing to do is to leave the marriage, not cheat on your spouse/partner. And if you can't keep in zipped, then don't commit to an exclusive relationship/marriage in the first place. But if you choose to do the morally bankrupt (IMHO) thing and cheat on your spouse, then you should be subject to negative consequences, even if it's only socially. Maybe, just maybe, it would serve as a deterrent to others.
As you know from experience, there is nothing better than a stable, loving marriage. Nothing on Earth. But there is nothing more painful than seeing your beliefs and dreams crumble to dust because of infidelity. I've been lucky, like you. But I've seen a dearly loved cousin trampled into dust by a cheating husband. I wouldn't wish it on my worst enemy.