I have found that none of the metaphors and similes work to try to explain this - whether it's children, works of art, cars, food... I have heard them all... - the response is invariably "Yeah, but this is different" and it's hard to argue against that without just falling into the "not it isn't", "yes it is" dynamic.
I think that GG's approach is the best - it expressed in no uncertain terms that you are uncomfortable, proposes a type of behaviour that you would regard as respectful, gives them the choice of behaving like that and continuing the relationship, or suspending it.
At some point you have to say "no more".
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