But hey, good luck drawing lines where you choose.
The ads speak for themselves: https://www.google.com/search?q=ashlymadison+ads
PS Those ads are awful.
I think the simpler explanation (and likely more accurate one) is that in most cases, each AM user has at least one partner which is being harmed by the user's actions and use of the site.
Being accepting and tolerant of different types of relationships and desires is a separate issue. The site was created for, and is marketed towards, people who want to cheat (in the traditional, harmful sense) on their current partners.
Secondly, still, you continue to make value judgments based on your own personal ethics and assume they're universal truths. Whether there is harm (as you say) truly is debatable. Ending a marriage is breaking your contract, doing something your spouse doesn't like is "harmful". Which ones are ok? Maybe this cheating for many people ends up being a one time thing that the person needed to do to either realize they want to end the marriage or realize they want to change something. Or some other outcome. Regardless, it isn't your or anyone else's job to decide, so please keep your moralizing to yourself.
I am not applying any overly austere morals here. I think you would be hard-pressed to successfully argue that an affair in the traditional sense is not an immoral act. If you promise to be "faithful" to someone, and you break that promise and try to hide it, it is immoral based on most people's morals and understanding.
Sure, you might disagree with popular morals, that's fine. However, a relationship involves two people, and both should be valued equally. If one person thinks it is fine to break their promise and cheat in secret, so they do it despite knowing how the other person feels...they are putting themselves first. They are not treating both they and their partner as equals. That is the issue. It has nothing to do with some sort of ideological anti-affair "beliefs." It is simple respect.
I guess it is even technically "fine" to think you deserve more liberty than others, or that you think others' feelings don't matter as much as your desires (I'm not saying you do, I'm speaking generally). If that is the case, just own up to it and people can choose how to react accordingly. If you find partners that align with your views, great, everyone is on the same page, but if not, people deserve the truth. Everything else is up in the air, but they deserve to know. Keeping it from them is like robbing them of the choice to deal with it or leave.
I guess promising to (not) do `X` and not keeping your word makes you untrustworthy but what is the literal harm on someone other than the self-referential belief that breaking promises is bad? To illustrate, let's assume that `X` is something like "I promise to not eat zucchini without you ever again". How is "cheating" by secretly gorging once on zucchini with some friends harmful to the person you give it to, other than their self-induced idea that it is bad?
Basically, it is irrelevant if they "should" or "shouldn't" be harmed, most of them will feel harm once the affair inevitably comes to light. I don't really agree with it, but even if it is irrational to feel harmed over an affair, and even if it is based on self-referential beliefs, people still feel the feelings, and those feelings hurt.
If the cheater knew those feelings would probably happen, and they already promised they would not engage in the behavior, and then they do it anyway...in secret...that is obviously wrong by most people's standards. There are a lot of important "if"s in there. Notice that I did not say that having multiple partners is harmful, or that leaving a partner is wrong, or that it is "always wrong" for someone in a relationship to go off with someone else for awhile. I did not say any of that.
However, when the relationship is a committed relationship, in the most common, Western, traditional sense, it is wrong to have an affair, especially a secret one. If you disagree with that kind of relationship and commitment, that is totally fine, but don't promise that to someone, and if you change your mind be up front about it.