I have no problem with veganism as a concept
The problem is the ones who are extremely vocal about it, these ones also (in my experience) tend to have no idea what they're talking about.
There was this one girl in one of my classes, she made sure by the end of the first day of the class, everyone knew she was a vegan.
She would say crap like "I don't eat gluten because it comes from genetically modified stuff which cause diseases"