No, they are not legally bound to it. Unless your marriage contract has some weird (and potentially invalid) clauses, you can't sue your spouse for cheating. It now won't even be taken into consideration when, for example, calculating alimony.
That depends on the state. Currently, in 21 states, adultery is still a criminal offense. It is very rarely prosecuted, but the laws are still on the books. In the majority of states, adultery can be taken into account to some extent in divorce settlements when determining division of property, alimony, or child custody.
You're correct that in some states adultery has very little standing in marital cases and divorces, but those principles vary significantly from state to state. It is still a generally established legal principle in most states that sexual fidelity is a general expectation in marriage. And it tends to be an official legal reason for divorce (in states that still allow divorce "for cause," rather than only "no-fault" divorces).
As for fidelity being an assumption, even though there's no real stats about it, the idea that a lot of people will cheat is well accepted. Because of that, still having this assumption of fidelity is pretty much wishful thinking.
Given that polls have consistently shown that 90+% of the American public believes adultery is "morally wrong," it may be "wishful thinking" for some people or some relationships -- but clearly it is an established social and moral expectation.
Infidelity does not cause direct harm. Any harm someone may feel because of infidelity is only in his own head. It's the same kind of harm some religious fanatics claim when someone "insults" their religion. We do not have a moral obligation to bow to anyone "sensitivities", no matter how they will be psychologically affected by the destruction of their illusions.
Every marriage is different, but at its basis it is an agreement about a relationship between two parties. As I said in the post you replied to, I have absolutely no problem with those two parties allowing sex with whomever if that's what they wish.
Again, the problem is deception and fidelity to the agreement. If you want to get married and have a clear and open agreement with your spouse that allows you to have an "open marriage," I have absolutely NO problem with that.
But clearly established legal opinion and the vast majority of public opinion believes adultery to be incompatible with the "default" position of a marital agreement. If you don't want a part of that, either don't get married, or be clear with your spouse about the fact that you expect an open marriage. Doing otherwise is benefitting from someone else in significant ways through deceit.
I can understand why you think a legal contract should generally be respected (of course sometimes respecting a legal contract can be morally wrong), but fidelity is simply not part of the legal contract.
Yes, it is in many states, as I already mentioned. But regardless of LEGAL circumstances -- there's also the morality of general agreements too.
Even if you don't have a legal right to sue someone for a breach of a particular contract, there is a notion of moral "fair dealing" and "honesty" in most contractual agreements. You posed your questions initially in moral terms, not only legal ones. Even if the law does not compel you to act in certain ways, morality may still dictate that certain courses are right and wrong.
Generally speaking, lying to someone to gain their affection (not to mention all the other benefits that generally accrue in marriages -- financial, social, etc.) is morally wrong.
What next? Will you say the obligation to cherish your spouse until death is also part of the contract?
Yep, at least the moral contract of marriage. Part of that "cherishing" is trying not to do things that your spouse would consider hurtful. If your spouse would consider an extramarital affair to be hurtful, then either don't cheat or find a different spouse who is more open to what you want.
Don't get me wrong, I will consider lying for personal gain as morally wrong, but saying banging someone else's spouse is morally wrong is only based on an idea of control, idea of control which in itself could be qualified as morally wrong.
Morality is defined by social standards, at least in part. As I cited above, most polls tend to show that the vast majority of people are in disagreement with you and believe that adultery is morally wrong.
Now -- if you want to change that, then by all means go and lobby public opinion on your behalf. Perhaps you are correct that monogamy and sexual fidelity is an irrational expectation, and perhaps we should work to change the fundamental nature of "marriage" in a way that would be more moral, to your liking.
But for now the default assumption in marriage is sexual fidelity. And if you sign the papers to get married, your spouse is probably going to expect that by default. You don't want that? Don't get married, or find a spouse who subscribes to your particular version of morality.
But to outright lie to someone you agreed to partner with for life (or at least semi-permanently, given today's culture of divorce)... sorry, I have no problem condemning that as morally wrong, along with anyone who voluntarily participates in such a deception.