I may have to do side stories, yes. I've developed some of their relationship outside the thread. (One person got a fair bit of it in PMs when we were discussing the fic.) There's just no time to devote to it in the main story, though. But yes, part of the reason the girls approached Kensuke was as a beard... but then he proved to be nicer than they expected, never pushed, and when they revealed they were also into each other, Ken just accepted it. He won a lot of points from them just by not either freaking out, or blabbing it to the world. He was fine with helping them maintain the masquerade. So they decided to give all the rumors they'd heard about a shot (sparked by Rei). Kensuke was a gentleman, and things started to snowball. Now they're quite happy they've found a boy who knows about them, doesn't mind, and cares for them both.
For his part, Kensuke is still half-convinced he's dreaming. Two girls that like him, and don't mind coming along his military geek LARP camp-outs? No way...