@ Eratas: By accident, such as tripping, etc.
If they haven't, then he'll think he got Tatenashi pregnant.
Hm, ship tease ideas... Kyuukai and Tatenashi lock the two in a sauna, clothed in only towels? Would make for an interesting moment.
Kyuukai should at the very least be bashful about his appearance. Not because of the guys there (he's not into yaoi), but because he's uncomfortable of all the scars lining his body.