There are many things that one does not have any sort of duty to accept. Of course it's not an argument; it's a statement.
Raping children is not acceptable. There are those who would argue otherwise. Saying it's not acceptable doesn't make anyone "lose the argument." The same is true about many things far less extreme.
What matters is the basis upon which someone uses "it's not acceptable." That determines whether or not the statement has merit. A blanket statement saying it has no merit itself has no merit.