Is defensiveness forbidden or is there a possibility to have an adult conversation?
Let's first rephrase this question:
Is it ever legitimate to defend oneself from attack?
Here's what Bill W says in his embedded essay on leadership under Concept IX:
Then we have those who we like to call our "destructive" critics. They power-drive, they are "politickers," they make accusations. Maybe they are violent, malicious. They pitch gobs of rumors, gossip, and general scuttlebutt to gain their ends—all for the good of AA, of course! Well, in AA at least, we have at last learned that these folks, who may be a trifle sicker than the rest of us, need not be really destructive at all, depending entirely on how we relate ourselves to them.
To begin with, we ought to listen very carefully to what they say. Sometimes they are telling the whole truth; at other times, a little truth. More often, though, they are just rationalizing themselves into nonsense. If we are within range, the whole truth, the half-truth, or even no truth at all can equally hurt us. That is why we have to listen so carefully. If they've got the whole truth, or even a little truth, then we'd better thank them and get on with our respective inventories, admitting we were wrong, regardless. If it's nonsense, we can ignore them. Or we can lay all the cards on the table and try to persuade them. Failing this, we can be sorry they are too sick to listen and we can try to forget the whole business. We can think of few better means of self-survey, of developing genuine patience, than these usually well-meaning but erratic brother members can afford us. This is always a large order and we shall sometimes fail to make good on it ourselves. But we must needs keep trying.
Applying that here, when I am accused of something, I accept what is true, reject what is false, and stick to the facts. There is no need to press my point. If it is not promptly bought, the buyer is not in buying mood, so I stop selling.
From A Course In Miracles:
It is essential to realise that all defences do what they would defend [against].
If I'm generally defensive, I create the suspicion and hostility that I fear.
This course can therefore be summed up very simply in this way:
Nothing real can be threatened
Nothing unreal exists.
Herein lies the peace of God.
In other words, I am not under attack so defence is moot. My conduct may be criticised, but my conduct is not me. Mistaking oneself for one's conduct (a breach of Tradition VI: lending one's name, i.e. identity, to something outside oneself) leads to all sorts of problems. If the person is perceived to be the behaviour, and the behaviour is unstable, no relationship is possible. It's hard to shoot a moving target. If the persons involved are eternal beings, the weather of their behaviour can come and go. It's important to recognise the substance behind the behaviour and relate to that.
Lastly, Bill W is very instructive in his essay on Concept XII:
Let us suppose that A.A. does fall under sharp public attack or heavy ridicule; and let us take the particular case where such pronouncements happen to have little or no justification in fact.
Almost without exception it can be confidently estimated that our best defense in these situations would be no defense whatever—namely, complete silence at the public level. Unreasonable people are stimulated all the more by opposition. If in good humor we leave them strictly alone, they are apt to subside the more quickly. If their attacks persist and it is plain that they are misinformed, it may be wise to communicate with them in a temperate and informative way; also in such a manner that they cannot use our communication as a springboard for fresh assault. Such communications need seldom be made by the Conference officially. Very often we can use the good offices of friends. Such messages from us should never question the motives of the attackers; they should be purely informative. These communications should also be private. If made public, they will often be seized upon as a fresh excuse for controversy.
In other words, do not defend. But quietly correct misapprehensions, if there is someone listening.