It's still happening today. Massive stink when one of the UK's undercover police broke down and admitted his guilt to the media in 2010:

http://en.wikipedia.org/wiki/Mark_Kennedy_(police_officer)

How many more of them are there? Who knows.

Note that it claims 22 different countries used him. And he was explicitly called out as being an agent provocateur.

It's important to note that his role was not just information gathering, but also agent provocateur, riling the group into criminal acts. Also worth noting that in order to maintain his cover he had sexual relationships with his targets... Which amounts to sexual intercourse by deception - also known as rape. Oddly, he was not prosecuted for this. Utterly shameful affair, and the absence of consequence indicates complete state sanction for his crimes.

Rape is not the same as "sexual intercourse by deception." The key variable is consent.

"Sexual intercourse by deception" could cover anything between having sex while lying about your age (definitely should not be illegal) to having sex while lying about your seroconversion status (often is and probably should be illegal). None of those count as rape, though, because consent had been granted before and during the actual act.

Lying about your long term intentions in order to have sex with someone, or using sex to manipulate someone? That makes you a bad person, but it's not illegal, and you have a really long line of people to arrest if you think it should be.