but we also have the right to expect a basic standard of decency and behaviour from people in society. People who don't behave to the expected standards usually find themselves in trouble with the law or without much of a friend/family support network due to not being trusted, and rightly so.
Why is knowingly embarking on an affair, as either the married party or OW/OM any different?
One, because it's outside the law, and so it should be. The state shouldn't interfere with people's sex lives beyond issues of consent and physical harm. Unless you do think it should be illegal for someone to have an affair in which case I can't discuss this with you, because you would support state control over people's personal lives to a frightening extent.
If you break your marriage contract, it can be dissolved on those grounds. That's as far as the state has a right to interfere.
If you treat people badly enough, you do indeed lose your support network. That's a natural consequence, up to you and them, and the law isn't required to intervene to make sure nobody wants to be your friend.
Two, similar to one, really: you can't expect the law to dictate that people can't be arseholes. If you want to make it illegal, joint enterprise, to have an affair, then it's also got to be illegal to lie about your income, your weight, whether you've had Botox or whatever to get someone into bed, because that's arsehoke behaviour. And it is arsehole behaviour, but you can't expect the law to interfere that far.
It's not illegal to be a total shit.
There's no such thing as joint enterprise when it comes to dictating to uncommitted people what they can and can't do, and a singular personal contract. Like I said, some people don't accept marriage as a valid institution and don't consent to be bound by it. They're allowed to live that way, as long as they don't marry. Once you do marry or commit yourself, you've made that personal pledge and you can't claim joint enterprise to hold everyone else in the world to it.
It's a complete fallacy based on a false equivalence.