Let the Cat Out of the Bag (2005)

A couple have become oblivious to their mutual indiscretions in this drama from German filmmaker Florian Schwarz. Karl will seduce women on a dare from his friends when he isn't committing acts of homophobic violence, while Doris has been having an affair with Ralf, a regular customer at the bar where she works. While Karl and Doris outwardly seem to have little interest in each other's sexual relationships, this doesn't prevent them from spying on their partners, sifting through their personal belongings when the opportunity presents itself. IMDB