I think infidelity is 'just another issue' that marriages face and not by a long straw the most serious - it's just the one we have been indoctrinated to get most outraged about. 'Sex' is the issue that the Church decided to focus on in formulating the 'contract', because sex has historically always been deemed somehow fundamentally 'sinful'.

Deceit comes in all forms, thus the 'contract' IMO is something of a secondary issue to *trust* and *honesty* between partners. And that trust can be compromised by all kinds of things apart from extra-marital sex: drugs, gambling, alcohol, debt etc. All of these things challenge trust and stability in a marriage. It just so happens that 'sex' is for whatever reason historically deemed more 'taboo' than other forms of deceit, and is thus written into the 'contract'.

I also suspect that most people do not enter into either debt, alcoholism, or indeed extra-marital affairs, in cold premeditated fashion whereby one could indicate ones prior intentions to ones partner. People are often weaker than they may think themselves to be, and can get caught-out by animal impulses before they realise how deep they are getting. To strive against such pitfalls is of course up to each of us. But I'm not about to get overly dismayed about the ordinary human failings of someone I don't know, unless it severely infringes upon another innocent parties rights or freedoms.