Like I said, if you want me to dive into why homosexuality is not a mental illness, I'd be happy to.
Why on Earth would someone choose to be gay? What (until maybe recently in certain large cities) are the benefits of "choosing" to be a homo?
If you were to say it isn't totally biological, I'd agree with that. Environmental factors play a role in shaping it too. That being said, the evidence doesn't support that orientation is a choice.
I'd be happy to have a conversation and dive into this, but it'll take me awhile to put some stuff together. If you're interested, I'll proceed. If not, then no worries.
Edit: Also, if it is a mental illness like you said, wouldn't that be an argument against it being a choice? So which is it in your opinion?