Tomo's partner is the school's beauty. One day, she goes on vacation, and he gets an interim partner who chides him for skipping foreplay. So that's why Rei's a dead fish in bed. Anyway, she comes back, and he finally manages to get her off.
In the epilogue, Tomo and Mio are dating. She wants to trade, but that'd never happen. Ever since that day, Rei's been all over him.