Saying "do everything properly" is an opinion too. Why is semantic markup more "proper" than Bootstrap's <div> and class names? The idea that semantic markup is a beautiful paradise is unsubstantiated. If you want non-humans to understand your markup you can use microformats, http://schema.org/ markup, or another form of microdata. Making "semantic" class names doesn't change how your users or search engines parse your content.
> People are getting things done with Bootstrap every day.
That is not an argument for correctness of approach. If there arose a cult of builders who decided to pour foundations with wheelbarrows and bags of concrete rather than a mixer, I'd tell them they're doing it wrong no matter how many houses they built this way.
The problem is that you can demonstrably point out objective flaws with using wheelbarrows and bags of concrete (or I assume you can - I know nothing about pouring foundations).
With the cult of semantic HTML, the flaws in using class names to describe presentation are almost always not explained beyond some vague notion of "correctness", and possibly an example that bears to relation to reality when you take into account that almost all HTML is being generated by some sort of templating engine anyway.
The problem with using wheelbarrows is that it will take a really really long time to get anything done.
"Semantic" css is like a cult of builders who sort the stones in their concrete by hand, and will only use the bluish stones for the foundation and the reddish stones for stairs. You point out to them that it simply doesn't matter, and they are shocked that you're not doing things the "right" way.