If I "internally feel" that I am fat but I'm not, that's called Anorexia Nervousa... A mental illness. If I internally feel that everyone around me is plotting my demise but they are not that's called paranoia... a mental illness. If I internally feel anxious even though there is no legitimate danger, that's called anxiety disorder. It's important to point out, in all of these conditions, the sufferers may be intellectually aware that their delusion is not real. For example, people with anxiety disorder often have these anxieties, the fight or flight response, while being completely aware that there is nothing to fear.... But they still feel that way. It's a mental illness. They know it's not real, but they can't help it. We don't help people with anxiety disorder by pretending there is a legitimate reason for them to be fearful.

When someone internally feels that they are male, even though they are female (or vice versa), that too is a mental illness. We do them harm by pretending it's true just as it would be harmful to pretend an anorexic is really fat.