Damn, the author change the way i see armpits now....then again, i've already seen enough Reimu "offbrand" with that tag to know where this is going. XD
@smashman42 iirc, they agree to do it at weekends and not everyday because:
1. If they do it, Sumika would definitely be tired the next morning
2. They both have their own jobs, especially Sumika as a teacher