I mean there's a difference between acknowledging that absolutes exist and dealing in absolutes. Obi-Wan's statement was a responce to Anakin's "Offer" of being subordinate to him or being destroyed.
It's more about the fact that, as a Sith, Anakin refuses to acknowledge the existence of the middle ground. It's his way or the highway, and anything that inconveniences him/anyone who has a different perspective is an obstacle to be eliminated without question- even, for example, his loving and heavily pregnant wife.