Is he your brother? / Not by choice.

I watched a movie 'Fifty shades of Grey' , if my memory serves me right.

This womanizer, Christian, is interested in a girl named Ana.
Ana is drunk at a bar, and Christian wants to get her home.
Ana gets to see a guy and asks Christian,

"Is he your brother?"

Not really wanting to answer,
Christian hurriedly leaves the bar with her after saying,

"Not by choice. Let's go."

What could he possibly mean by that - not by choice?

When asked that kind of question, people normally would answer,
'He is Tom, two years younger than me.'
'His name is Jack, the oldest brother in my family.'

or 'No, he is not my brother'
if he really isn't.

But 'not by choice'?

I pondered to jump to this conclusion.

The parents didn't want him, but they failed to practice contraception,
which resulted in his birth.

Christian knows this.
So, he probably meant he came out to the world against the parent's will.

I've got this gut feeling that my guess isn't very correct.
Would you please help a S. Korean having troubling understanding that phrase?

Thank you.
Happy New Year to everyone.