From (Season 5 Episode 9) «The Last Temptation of Homer»
Mr. Burns hires a female worker named Mindy Simmons in accordance with government policy and Homer is worried that his crush on her will ruin his marriage with Marge. Both Mindy and Homer have feelings for each other, but in the end Homer decides not to cheat on Marge. Meanwhile, Bart becomes a nerd after doctors find several things physically wrong with him and apply treatments that make him look like a nerd.
source