I always figured that he was doing it because he was secretly a fan of yuri which by the way, would also explain why he didn't make a choice between Sheryl and Ranka in the end - by having them both, he's able to experience both being an oné-sama (to Ranka) while having an oné-sama of his own (Sheryl). Off course, for this alone I can conclude that Alto is very manly, I mean come one, what other guy can claim to be this close to being in a lesbian three-way.