There is no single answer, because "cheating" is defined by the people in a relationship, not by the technology. What matters is honesty and whether you would be comfortable if your partner knew.
Start FreeIf you feel the need to hide it, that discomfort is worth examining. Many couples would treat an AI companion as harmless entertainment; others would see secret intimacy — even with software — as a breach of trust.
Be upfront. If you would not mention it, ask yourself why. For many people the honest conversation is more useful than the app.
It varies. That is exactly why the conversation matters more than a general rule.