Let's get one thing out of the way first: I am a straight white male. But the truth is, nobody's "normal". If ANYONE was "normal", they wouldn't be interesting or special beyond their value in their own family. I have several friends who are LGBTQ, and I am okay with that. In fact, I strongly support them, because they ARE normal--as normal as one can be. They're still human (despite popular opinion), they need food, they need water, they like doing regular things, they go to school, and most of them are better people than me. They are normal by human standards, and this question makes me angry because it shouldn't be a question: of COURSE they're normal.
People who believe this have a mental illness because they are not the way that they were scientifically meant to be. There is a reason most people are straight. Anything else is a mental illness. It is unfortunate that some people are that way, But they still have a problem.