The Judge from the Ace Attorney games falls into this trope many, many times. From case 5 of the first game, this is his response to being told that a witness was using another character as his "puppet"...
Oh you mean someone who uses someone else, nevermind.
Masayuki occasionally in A Profile, though it's not always played for laughs. But one that is goes like this:
Masayuki: So, you have multiple personalities, huh? Awesome, I can have a threesome this summer!
Shirou in Fate/stay night has this problem when any of the Deus Sex Machina problems come up, as well as at the end of UBW good ending. The reason for this is quite complicated and only elaborated upon in other material. Basically prana in the form of od the magical energy needed for magic tends to concentrate most strongly in a mage's body fluids namely blood and semen (Word of Dante usually brings up vaginal fluid) the former of which is responsible for certain acts of vampirism like those performed by Akiha Tohno. And it keeps (as in the magical power doesn't fade) so impoverished mages for instance sell their blood as a kind of mana potion and it's implied that some other mages sell sex for the same reason. This is apparently common knowledge among mages except for Shirou who has never actually associated with any mages other than Kiritsugu.