He realises that she will never understand.

TREBELL.  My dear girl, I’m sorry to be brutal.  Does it matter so much to you that I should have wished to be the father of your child?

AMY. [Ungracious but pacified by his change of tone.] It doesn’t matter now.

TREBELL. [Friendly still.] On principle I don’t make promises.  But I think I can promise you that if you keep your head and will keep your health, this shall all be made as easy for you as if everyone could know.  And let’s think what the child may mean to you ... just the fact of his birth.  Nothing to me, of course!  Perhaps that accounts for the touch of jealousy.  I’ve forfeited my rights because I hadn’t honourable intentions.  You can’t forfeit yours.  Even if you never see him and he has to grow up among strangers ... just to have had a child must make a difference to you.  Of course, it may be a girl.  I wonder.

      As he wanders on so optimistically she stares at him and her face
      changes.  She realises....

AMY.  Do you expect me to go through with this?  Henry! ...  I’d sooner kill myself.

      There is silence between them.  He looks at her as one looks at some
      unnatural thing.  Then after a moment he speaks, very coldly.

TREBELL.  Oh ... indeed.  Don’t get foolish ideas into your head.  You’ve no choice now ... no reasonable choice.

AMY. [Driven to bay; her last friend an enemy.] I won’t go through with it.

TREBELL.  It hasn’t been so much the fear of scandal then—­

AMY.  That wouldn’t break my heart.  You’d marry me, wouldn’t you?  We could go away somewhere.  I could be very fond of you, Henry.

TREBELL. [Marvelling at these tangents.] Marry you!  I should murder you in a week.

      This sounds only brutal to her; she lets herself be shamed.

AMY.  You’ve no more use for me than the use you’ve made of me.

TREBELL. [Logical again.] Won’t you realise that there’s a third party to our discussion ... that I’m of no importance beside him and you of very little.  Think of the child.

      AMY blazes into desperate rebellion.

AMY.  There’s no child because I haven’t chosen there shall be and there shan’t be because I don’t choose.  You’d have me first your plaything and then Nature’s, would you?

TREBELL. [A little abashed.] Come now, you knew what you were about.

AMY. [Thinking of those moments.] Did I?  I found myself wanting you, belonging to you suddenly.  I didn’t stop to think and explain.  But are we never to be happy and irresponsible ... never for a moment?

TREBELL.  Well ... one can’t pick and choose consequences.

AMY.  Your choices in life have made you what you want to be, haven’t they? 
Leave me mine.

TREBELL.  But it’s too late to argue like that.