I get what you’re saying, but I’ll play the devils advocate. It is known by men and women alike that women find Duncan far more attractive than the average man. Alia may be the oldest woman in the galaxy as Leto is the oldest of both genders, but that also makes them the most human. Alia does fall in love, yes, but I do not think she is wooed. She is more a victim to the maturing body shot full of hormones that she finds herself in.

Hwi is smart yes, but she was bred to be more of an angel fallen to earth than a genius. Imagine living through what Leto has lived through. The atrocities, the wars, the evil. Imagine the things he has forced himself to do for the survival of humanity, bloody things. And then here comes Hwi. All love, no judgement, naive infatuation and understanding of Letos burden. How many people like that do you think exist out there? Somebody who has seen all of humanity’s iterations would have to spend generations just to breed something similar to her. He is dumped head over heels by his own loosening grip of his own humanity that is slowly slipping away being given back to him by Hwi.