There's a black hole sized gulf between "perhaps there is a reason for homosexuality" and "homosexuality is caused by Teflon and micro plastics"
If you aren't going to even attempt to point at something that even comes close to backing up your claim then you're going to be very disappointed with the amount of people who take you seriously.
Why would you put "by definition" in there, that changes it from a "this is my opinion" to "It are a fact, i know because of my learnings".
It's possible there is a definition somewhere that specifically references mental health i suppose , i'd be interested to see it if you have a link ?