The only way I would EVER consider crossdressing a "mental disorder/disease", is if it is fully damaging the person and the people around them.
Let's say someone is having conflicts with their male morals and their crossdressing side. It's fine and all, we all go through it. But, if that person is having severe break-downs and it is debilitating their entire life, then yes, that person should be seeing someone (on their own, not forced). That is when it becomes a disorder (dis-, order)
Or, taking it into another realm, let's say that someone is having sexual fantasies with crossdressing. Fine and dandy, we all go through it. But, if crossdressing is the only thing that makes them sexually climax, then they do need to see someone about it (on their own free will, not forced). That is also when it becomes a disorder (dis-, order).
Laws, rules, and regulations should NOT be pawns for others' disgust and/or opinions.