[tomku]

There are two separate issues here, adultery as a crime and adultery as a breach of contract. I'll talk about US laws, since they're the ones I'm most familiar with.

Adultery as a crime is still surprisingly common, but going away fast. Prosecutions for it in Western countries are extremely rare, and the laws making it illegal in 23 US states are just waiting to be challenged and overthrown. Most people seem to think that it's not the government's business to decide that all husbands and wives must remain faithful or be fined/jailed.

Adultery as a breach of contract is a little trickier, because a marriage contract is a special kind of contract that covers things that normal contracts can't. You can't sue someone for breaching their marriage vows, but you can divorce them. In the past, you needed grounds (such as adultery) to do so, but no-fault divorces are available now in every US state and several other Western countries. Whether adultery is factored into decisions about alimony depends on the jurisdiction - some do, some don't. It's a tricky issue, and relationships (particularly rocky ones) tend to be more complex than rigid rules can allow for.

On a more general point, I think that you'd have a hard time convincing a judge that lifetime sexual exclusivity is something you can sign away in any normal legal contract. Marriage gets special treatment to allow for things like that, but the trade-off is that we expect married couples to try to deal with their own issues rather than getting the courts involved immediately.