Digression into a bunch of theory and science impersonalizes things as well as focussing on 'me' instead of 'you'

Not really. Any evolutionary explanation of why I am repulsed by your physical appearance is going to spend a lot of time dwelling on your physical appearance. And I think the impersonalization bit is the key - it is a ridiculously impersonal digression at a moment of extreme emotional vulnerability on the other person's part. Most people will interpret impersonal explanations of this sort of emotionally impactful decision as an extremely cold-hearted way of excusing oneself. "I'm sorry I've just hurt your feelings. But allow me to explain how this is all just the work of the forces of sexual selection in our ancestral environment..."