Annoyed
Ryu's expression was a bit weird. His reaction probably wasn't what Selheira might have expected, only because he didn't know the truth behind what was happening now. For all he knew, this was Aika's choice. Truthfully, she did seem like the type. Since she hadn't found a husband until now, picking out several fit with her archetype, at least from what he knew about it.
It wasn't exactly unheard of either. Take the Martial Gods for example, they were an exceptionally powerful Clan and their rules on such things were far more lax than many others. Isemeine's own mother had two daughters from two different men, and they both fought over her just the same.
And if Ryu was correct about the Martial Gods, they were an existence amongst the Martial Powers of the Ninth Heaven, which meant that this was the standard they upheld for themselves even in those highest of echelons.