Degradation is definitely the main one for them, even if Horatio smarts against it (he wants LOVE!!!!) But he still becomes putty in Elethar’s hand whenever Elethar spits on him or tears him down. Elethar loves to watch him squirm. The dom/sub dynamic is critical.
Another thing I’ve been considering lately (and this is like. WAAYY too horny so I’m sorry in advance) is that Horatio has been ummm... impacted... by his family’s constant demands for an heir. His most deeply buried desire (which, as you said, he wasn’t even aware of until they started fooling around) would be to have Elethar’s child 😳 like to that end he kinda fantasizes about having a pussy but not in a transgender way just in a like ‘wow this man is truly off the deep end of infatuated’ type of way. He craves that family life. Well brother that is not going to work for so many reasons and you having a pussy is not about to solve any of them