It's best if you expand on what you mean by choice/mental illness.>Mental Illness
Being gay doesn't effect your ability to live and work in society, you can be gay and still run a Fortune 500 company. Thus it can't be considered a mental illness. >Choice
You can choose to eat an apple or a pear, but you can't "choose" to be gay in the same way you don't choose to be straight.
In a sense that you wake up one morning and consciously think "I've made up my mind, I don't like the other gender, I like my gender".
They wake up, unexpectedly see a hot person of the same gender and start to develop sexual feelings and thoughts without purposely wanting to.
But does that mean we should just blindly accept someone's gay and leave it at that? That's the question we should be asking.