Earlier quoted context omitted.
This is not a factual statement. It's your opinion. It isn't up to you to decide this for other people.
Its intent is to do something which harms. I don't think the infinitesimal collection of people who want their partners to "cheat" on them justify the flexibility you're implying. Basically, what you are saying, is that some people are not harmed by being cheated on, and therefore it isn't fair for us to judge the actions of the sites users, because maybe all or most of them are in a situation where there is no harm.…
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?