Being gay, or bi, is not a mental illness. If your argument was that gender dysphoria (transgenders) is a mental illness, I'd agree with you, but that still wouldn't mean that it'd be okay to disrupt their lives and take certain rights away from them. Also, I feel like it has to be pointed out, having a mental illness isn't a "bad" thing, it's not some insult, it's a harsh reality, a reality that requires some level of care and understanding. If you think picking on the mentally ill is something you should be doing, then I think you might be mentally ill yourself.