What is the theme of "Hills Like White Elephants" and a point that supports that theme?

The shadow of a cloud moved across the field of grain and she saw the river through the trees.
‘And we could have all this,’ she said. ‘And we could have everything and every day we make it more impossible.’
‘What did you say?’
‘I said we could have everything.’
‘We can have everything.’
‘No, we can’t.’
‘We can have the whole world.’
‘No, we can’t.’
‘We can go everywhere.’
‘No, we can’t. It isn’t ours any more.’
‘It’s ours.’
‘No, it isn’t. And once they take it away, you never get it back.’
‘But they haven’t taken it away.’
‘We’ll wait and see.’
‘Come on back in the shade,’ he said. ‘You mustn’t feel that way.’
‘I don’t feel any way,’ the girl said. ‘I just know things.’

Several important themes are choices and consequences; doubt and ambiguity; and men's perspective versus women's perspective. To my mind, however, the most important theme is honesty versus dishonesty.

The foremost instance of this theme is that the American man persists in saying that he only wants Jig to undergo the operation if she wants to yet, at the same time, he persists in claiming that it is a simple and perfectly natural procedure and that he is sure she wouldn't even mind, since it really is nothing. One of these sets of expressions of sentiment and opinion is dishonest. Either he is putting her wishes first or he is putting his wishes for what her feelings and experience will be first. Both can't be true at one and the same time.

'I don’t want you to do anything that you don’t want to do -’
‘Nor that isn’t good for me,’ she said. ...
‘You’ve got to realize,’ he said, ‘that I don’t want you to do it if you don’t want to. I’m perfectly willing to go through with it if it means anything to you.’

Another example is that he presumes to fathom what a woman's feelings and experience will be in a realm of life that is exclusively female. This too is dishonest. Honesty would require an admission of limited perspective and empathy. Honesty would require the courage to refrain from trying to shape Jig's sentiments and feelings. Honesty would require an unveiled, unambiguous expression of his wishes, which--by all evidence in the text--is that he wishes to not be the father of a living child.

'I’m perfectly willing to go through with it if it means anything to you.’
‘Doesn’t it mean anything to you? We could get along.’
‘Of course it does. But I don’t want anybody but you. I don’t want anyone else. ....'

Finally, the conversation excerpted above further expresses the man's dishonesty. Jig is expressing her perspective of the finality of the circumstance. On one hand, if she has the abortion, her world will be changed forever since an abortion is not an insignificant thing, either physically or spiritually. On the other hand, if she does not have an abortion, her world will be changed forever but in a very different direction: she will lose the frivolous and fun relationship she has with the man--as they travel and collect luggage labels and taste new drinks--and she will have his child to mother. Dishonesty is represented because the man won't admit to the change in dynamics the pregnancy brings into their relationship as a result of the change in dynamics it brings to Jig's life.