Milkman has performance anxiety and is planning to spare them both an embarrassing performance blowing his load in solace. Kokoro decides to put on a bit of a solo act for him to get him in the right frame of mind to continue.
Not loved this chapter but of course I'm curious about the next XD.
The love story between Mitsuru and Kokoro is much better than anime, Mitsuru is always an idiot because it is precisely the characterization of the character to be so.