Simply say "I am gay and believe that God is okay with my homosexuality. I understand that you feel I should repent but hopefully you can see that I cannot do as you wish because I believe differently to you. Perhaps we can agree to disagree?"
In that you have presented your case, indicated that you have made some effort to listen and understand their point of view and suggested a positive, reasonable course of action. They can't really expect anything more from you than that and if they do then that is their problem, not yours.
If they retort with anything insulting then just smile at them and say that you are very sorry that they feel that way.
Convincing them that your lifestyle is not chosen is very difficult because their belief that it is will likely not be based on evidence or reasoning or at least not anything that you can create a convincing argument around. I tend to find that you get more out of a conversation if you remain open to the possibility that you are wrong and hope that whoever you converse with does you the same honour. You then spend less time being frustrated at their accusations, no matter how unjustified they may be, and more time trying to understand their point of view. Generally, this makes it far less upsetting, for me at least.