Fantastic. You did it. A human/centaur story that is absolutely believable, round, affectionate and hot, and includes not only the relationship but also the people around them -- not an easy feat, I'd imagine. And a great read.
I love so many things about this. Dudley the bisexual one-centaur man. Adelaide. The whole "physicality" bit, how sex and fighting go together for Dudley, and that it's that aspect of physicality, rather than a person's gender, that makes someone sexually attractive for Dudley. And, last but not least, your portrayal of Petunia and the closure you give her. Her relating to Lily put a tear in my eye.