- I think some soft slave play might work really well. Have Asterion really want to pamper the MC, play up how he's always been trying to put himself in a subordinate role, let him really go wild with that. Maybe make Asterion have to talk MC into letting him do so, especially if they've made lots of "stop calling me master" choices. There's ways to eroticize the MC's ability to give "orders" and (on top of being HOT) it's a not uncommonly reported method of taking agency over past abuse, to turn it into kinky scenarios (though it should be emphasized that this isn't an approach that automatically works for everyone.)
- You could use the "control over Asterion's body" to some interesting effect, I think. Maybe as simple as a "...so what happens if I just ORDER you to become horny? Does that work?"
- I can see Asterion spouting extremely elegant and extravagant, to the point of being a little self consciously silly, poetic praises of whatever part of MC's body his lips happen to on right now.
- 'Sauna' is definitely good. But I'd also consider having horny possible extensions to already existing R&D projects, if they are completed AFTER the relationship becomes sexual. The Gym and the Pool come to immediate mind as good possibilities.
- I think Asterion might conceivably be uncomfortable with a sling/sex swing, or anything which might remind him of the cow Daedalus built Pasiphae.
- The scene where the MC examines the nose ring, and handling it put Asterion almost into a trance, seems me to be foreshadowing horny applications. Like, gentle pressure down on it to get him on his knees, for example.
- Asterion's got far too much chest and torso in general to not be able to use it to good effect. Like just bury MC's face between his pecs.
- I think Asterion's the kind of person who falls in love with someone over a shared task (like, well, running a hotel) But like, the kind of "collaborators to friends to lovers" path. So i think he'd like some activity that then turns into passionately making out. I could see him getting the kind of performance anxiety that comes from having to initiate sex from like a cold start, as it were. I could see him much preferring sort of semi-spontaneous "oh you're brushing my back, oh, something about this feels good..." kind of scenario. Similarly, I could see him liking scenarios where he doesn't have to think about things, doesn't have to make decisions, where he can just be told "use your physical strength and sexy man-body in such and such a way." He'd be submissive because he's got the kind of personality that feels the guilty and therefore more uncomfortable with the sex the more responsible for it they are, so he'd like someone else to call the shots.