A person - typically a man - who wishes to receive oral sex so he tells the female he will perform the act on her as well, in order to convince her to give him oral sex. Once she has given him oral sex he declines to give her oral sex, and either proceeds to regular intercourse or says, "I owe you one," while never intending to give her oral sex. This term can also be used to describe a partner who gives quick and insufficient oral sex, short-changing his partner. Based on the sex position "the 69" where both partners give and receive oral sex simultaneously. In this case since one partner got an "I owe you one" it is 69 minus 1 or 68.
Person One: "Never date that guy! I heard he is a 68er!"
Person Two: "Believe me, I won't. 68ers are the reason so many women are enjoying lesbians."