S7·E21 The Why Chromosome

When the men are nowhere to be found, Sheppard and Ballard team up to investigate the death of young girl, who's a teenage gang member. With the wedding one week away, Billie Lou asks for Munch's help with her neighbor's abusive boyfriend. After Munch approaches him strongly, the boyfriend retaliates by coming to the Waterfront and slamming Billie Lou's head on the bar. Munch claims he came on strong with the guy, because his ""testosterone riddled behavior"" was due to the celibacy Billie Lou imposed on him, to make their wedding night more special. Billie Lou wants to delay the wedding until the welt on her forehead goes away in about 3 weeks. After Sheppard and Ballard solve their case, Sheppard reminds Lewis that his problem with her is not going to keep her from doing her job.