He said that under the Regulation of Investigatory Powers Act 2000 (RIPA), officers were permitted to have sex as part of their job but the legislation meant the operations were strictly managed.
There had been confusion about whether undercover police were allowed to go that far following the collapse of a case against environmental activists in Nottinghamshire after it emerged the group was infiltrated by an officer called Mark Kennedy, who had been in sexual relationships with two women in the campaign.
Mr Kennedy spent seven years posing as long-haired dropout climber Mark "Flash" Stone to infiltrate activists and admitted having sex with at least two women during the operation.
Today, Mr Herbert said it was important police were allowed to have sex with activists because otherwise it could be used as a way of outing potential undercover officers.
Speaking in a debate in Westminster Hall, Mr Herbert said: "In very limited circumstances, authorisation under Ripa Part 2 may render unlawful conduct with the criminal if it is consentutory conduct falling within the Act that the source is authorised to undertake.
"But this would depend on the circumstances of each individual case and consideration should always be given to seeking legal advice.
"I am not persuaded that it would be necessary to introduce specific statutory guidance on the circumstances of sexual relationships under Ripa.
"I think what matters is that there is a general structure and system of proper oversight and control rather than very specific instructions as to what may or may not be permitted.
"Of course, there is another point that banning such actions would provide the group targeted the opportunity to find out whether there was an undercover officer specifically within their group."
An HMIC report found Mr Kennedy worked outside the code of conduct for undercover officers, became "resistant to management intervention", and review and oversight was insufficient.
"He seems to have believed he was best placed to make decisions about how his deployment and the operation should progress," the report said.
Mr Kennedy worked undercover in 11 countries on 40 occasions, mostly on "European-wide protest issues", but there was no single officer in control and the authorising officer was not even always told Mr Kennedy was going overseas, nor given relevant information about what happened while he was there.
Mr Kennedy also defied instructions and went abroad with a protester in 2009 and carried on working against instructions despite being arrested in 2006.
