Help a reader out :D

I had a question about the BDSM in the Spotless series-- how hard is the BDSM in it?

What would y'all say about it? There's none in the first book, things get a little rougher in the second but not in the way people might think, and the third one doesn't contain BDSM either. Opinions? Help? What would y'all say? Thank you!