Honestly, I feel like Asterion -> Themba would be better under that justification than Robert. Ignoring the whole familial vibe they got going on with each other (which worked out fine for Kota anyway), Themba would likely be the most receptive of the bunch to being called sir and being looked up to with respect. Plus, as someone else in the thread said, Themba is a confident kinky top and Asterion is an open-minded, receptive, and curious bottom. We also already know that they can form an emotional bond on some level from the whole familial thing, if we do wanna bring that into the mix.
My logic was that Themba has too much respect for Asterion specifically. I figured part of it is because Asterion helped raising him. So, while they should be sexually compatible, Themba could have to dismantle an internal hierarchy established during his childhood. By internal i mean that Themba considers Asterion his superior even if that's not really the case at all, and obviously Asterion would never enforce it.
Aside from that, there is another reason i have for this. I already experimented with that thought with Themba -> Asterion. In a sense, i matched each character twice. You could say i "cheated" to have more pairings, but that's because i wanted to force myself to think of more potential scenarios.