easier way, given Rikka's personality is just to tell it to her straight. she approaches her along with Souta, confesses that she is cursed. confesses that she may or may not love Souta, confesses she had Souta satisfy her urges. Then let Rikka Decide. they could all go the harem route cause its the "Curse's Fault" or something. Or better yet...if they think more and Usui's own feelings be damned, then she could just go for another guy when she's pent up. then if that works that guy will be her boyfriend then.