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.