Unbeknownst to most, Master Roshi teaches both martial and sexual arts. While his star pupils - Goku and Krillin - showed little interest and little talent (respectively) in the later, some would take all of his teachings at heart : body training to enhance one's stamina, ki control to harden beyond the body's limits, ki aura to arouse the partner, ki projection to induce ovulation...
With C18 living with her boyfriend Krillin on Roshi's island, one of the most experienced sexual arts students of the perverted ermit found the perfect target practice🍆👌😩💦
(Trivia: C18's bull is wearing the same glasses as Maste Roshi in Dragon Ball Z, which are red and green. That particular red is called "tortoise shell" [used a lot for glasses frames], which I find funny considering that he is Kame [turtle] sennin)