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The only rules in marriage are the ones you and your spouse agree to. Some couples consider it cheating as soon as you simply flirt with another person, while others are OK with their spouse having sex with other people. So whether it's cheating or not is purely a function on the couple involved.

Cheating does not depend on physical or lifelike.

Cheating is when you say "I love you with all my heart" but in the mean time give your heart to other stuff.

Or maybe just humanlike? It may be that the uncanny valley makes people prefer their bio bots to be just like humans - except without heads.