Can't agree. Do you really allow irrational people to dictate your behaviour?
Not me.
I'm not suggesting you have to accept it - I sure as hell wouldn't - but you do have to respect their feelings on the matter, their set boundaries, that that's how
they define cheating. And they're entitled to define cheating as they see fit, as determined by their own comfort level, sense of security (or insecurity), religious beliefs, or whatever. Just as you're entitled to say they're being irrational, unreasonable, or batshit crazy, and walk out the front door and run like mad to get the fuck away from their crazy ass as fast as you can.
My point is, you can't/shouldn't dictate to your partner how they need to define cheating, how they should feel about this or that act. It's an extension of sexual compatibility, and either you're on the same page or you're not, and either you choose to accept the differences and try to work within those confines or you get the hell out of the relationship. Because, yes, I think it's bullshit and irrational to say "Watching porn is cheating!" or "Inserting anything in your vagina or anus that isn't my cock is cheating!" But it's just as bullshit and irrational to tell someone "
I don't consider it cheating, so you shouldn't either and you should stop being upset!"
You can discuss it all you like, try to bring them around to your way of thinking, and maybe you'll come to a compromise or a satisfactory resolution if you can address why they feel the way they do and give them the assurance/confidence/understanding they need to move past it, but at the end of the day, if your partner doesn't want you doing something, you either have to respect their wishes and not do it, or you need to end the relationship and chalk it up to sexual incompatibility/irreconcilable differences.