"There are things which one just does not want to see" is what Seiichi thinks upon seeing his big brother Senri making out passionately with Seiichi's own art teacher.
"Two men are also able to...?"
Seiichi is obviously confused, but his brother Senri means: Only exposure therapy can help! Which means round two for this kind of visual instruction. And while Senri and his lover slowly get into motion, Seiichi starts to watch with more attention...

(Source: Roughly translated from official German cover)