Of course it's a mental disease (well from the point of view of biological reproduction). It is abnormal for a human to be anything but straight as it is counter-intuitive to the basic survival of the species. Before anyone get's all anal about the word 'disease', it isn't necessarily an evil, vile word, but people tip-top around it because of the negative connotations. I don't think there is anything wrong with homosexuality but that doesn't mean it isn't abnormal. I also think wanting to fuck inanimate objects is a disease but it's pretty fucking harmless, right? Though it's semantics people will be offended by all language and they create their own nomenclature. Though, I do suppose I'm looking at it from an entirely fundamental perspective, reducing humans to the most basic and primitive state. My definition doesn't really apply to western-society.
https://www.merriam-webster.com/dictionary/disease
'a condition of the living animal or plant body or of one of its parts that impairs normal functioning and is typically manifested by distinguishing signs and symptoms'