It is very rare that you find someone in life which has no negative side effects. You can try to limit these side effects or maybe find a slightly better alternative, but there are still going to be negative effects. Sustainable tourism is just the same.
While sustainable tourism is one of the better alternatives to general tourism because it helps limit the negative effects on the local communities and the distribution of jobs and the local economy, it still has some negative things they have not solved yet. One of the biggest issues is travel. It is just impossible currently to provide air or car transportation that is sustainable.