It's not a choice at all. It's how you are born.

The choice you have is whether or not you repress your sexuality, not whether you're straight, bi or gay. And to repress your sexuality will only cause you problems later on in life. I've seen it happen.

He's wrong it's not a choice.

However, you still want to be friends with him so you'll have to agree to disagree.
Think of it from his perspective, he is probably Christian and believes that you being a practising lesbian is against God will and a sin. Thus, he doesn't want you to sin and incur Gods wrath so telling you his opinion and that you should exercise self control is in his own way him doing you a favour.

He probably honestly wants what's best for you as he is your friend.

I suggest you don't bring it up that often and concentrate on things that you do have in common. And understand that its more important to understand and respect his point of view that to have him change his mind. Because understand and respecting his point of view is the only way you might change his mind.