I can't help but think that, considering how these two feel about each other, and that they're married, being in such close proximity with one another each and every night, instinct would begin to take over; even if it's clumsy, he needs only to look into her eyes once to know that she's so horny it hurts. It's been at least two months now?
Chiyo really is an unusual girl: she's a beautiful combination of perverted and pure; almost every man's dream. Even though she seems to have no knowledge of the basics regarding sex, in realizing her sister's attitude, I'm pretty surprised that some info wasn't shared before the wedding night. Soujirou just needs to tell Chiyo to speak freely, all her concerns; their antics are entertaining, but their communication levels are atrocious.