Well I can't speak for anyone else, but for me its disrespectful because its not what its purpose is. And its a rule in our house. Maybe its because DH and I are older parents and not in our 20's, but we were certainly never allowed to do that in our house, and as "wrong" as it may be, both of us follow society's rules and are happy with the job that our parents did with us. So I guess we have the same rules in our house now as we had in our house as kids.
Plus, its DH's and my property that they are effectively destroying when they jump on it - and I don't want them to go somewhere else and jump on someone else's couch. Which is what they would do if we let them jump on ours.
So we start respect for other people's belongings in our own home. And if thats seen as being disrespectful to the kids by not letting them have some fun or whatever - well - I don't really care to be honest what other people think, unless they are willing to buy us a new couch PMSL!!!!
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