According to former agents who worked in a secret unit of the British Metropolitan police, when undercover police officers — both male and female — were infiltrating anarchist environmental groups, the police routinely adopted "a tactic of promiscuity" in order to capture their enemies. Specifically, they slept with them. Sure, plenty of people have cop fantasies. However, the cops were only sanctioned to sleep with the left-wing anarchists they were infiltrating, not with any old suspect. According to police, left-wing anarchists "have an extremely promiscuous lifestyle," so officers just had to sleep with them to in order to fit in while undercover.
Now some people are unhappy about this, including the left-wing activists being duped into sex with undercover officers because they had a reputation. The Association of Chief Police Officers denies the report altogether, saying operatives were forbidden from sleeping with activists. But according to several former agents, the only thing forbidden was falling in love with an anarchist. Now that would be unprofessional.
Undercover police cleared 'to have sex with activists' [Guardian UK]
Undercover PC who married activist on restricted duty [Independent UK]