To be honest, besides how flustered and embarrassed they get, I've come to the conclusion there is a very good reason they shouldn't be jumping to the big act that is sex, and it has to do with Tsukasa's heavily-implied immortality at this stage in the narrative. Way I see it, the way she likely gained it, and its effects, probably make doing it either excruciatingly painful the whole time, or virtually impossible for one reason or another. They really shouldn't be trying to do it until Nasa can figure out how to un-immortal her. This also means he needs to be aware of the problem, but it hasn't come time to reveal the secret, even if he might have an idea or two at this point.
But it's not particularly pressing right now. They're enjoying a relatively relaxed life of co-habitation dating as they gradually grow more comfortable with each other. It's only been a month since this relationship started, after all, and they started it with barely knowing anything about each other and no prior time spent around each other even as acquaintances, let alone friends. Give them grace to advance at their own pace.