This is Tsuki Wani's "No Experience With men" series in which some of Gensokyo's Old Maids are revealed to have ridiculously little experience in sexual matters (especially with men). Its a frequent fan-gag to point out that Mystia's singing ('chin-chin' is the onamaetopia for bird chirping) is also slang for "d*ck", so Mystia is basically repelling the sexually shy Yuyuko by singing "D*cks, d*cks" again and again.
