That's not what I said though.

Of course, no one is under any obligation to stay with anyone else, ever.

But, for a cheater to be the one that breaks the trust, and then say "if you don't trust me, you should break up with me instead of spying on me trying to discover the truth", then that's manipulative. The cheater, in that scenario, wants the best of everything: the benefits of infidelity, not getting challenged or being proved to be a cheater, and not having to initiate the breakup themselves, so they can reap the benefits of both relationships as long as possible.