I suppose we could see it (the necessity) as an implication of him saying that it is permitted.
Yes, it's definitely implied.
All necessary things are allowable, but not all allowable things are necessary. Yet, some allowable things are necessary, in certain cases, else the allowance wouldn't exist.
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I get the general gist of your argument, but i'm debating some of your generalizations, which aren't correct. But I do understand what you're getting at, with your generalizations. Obviously, those you are debating are interpreting general moral theology in a different way than you and I (and others). This is why moral theology is difficult to debate - everyone agrees on the principles, but have different conclusions.