That '70s Show (1998) - 6x17 - Happy Jack

Donna catches Eric spanking his monkey in her bathroom, much to her chagrin. After wards Eric's friends and family tease him mercilessly, and his girlfriend won't even look him in the eye. He blames it on the no-sex-until-we're married vow she roped him into...but when he lets spill that his need was brought on by pictures of Jackie's mother, matters get worse. After a talk, however, he realizes that what he did wasn't so horrible--and girls do it, too (and apparently better). Meantime, Kelso--now calling himself "Mike" and sporting a mustache--goes on a major maturity kick, and Hyde and Fez have to knock him down a peg or two.