Joe Ashton, the Labour MP for Bassetlaw, told the Home Secretary at a backbench Labour meeting in the Commons that he was concerned about a proposed free vote to reduce the age of consent for gays from 18 to 16 years.
Mr Ashton said: "I am keen to stop older men from abusing their authority. It is not gay sex between 16-year-olds that is the problem.
"It is the enthusiasm of 45-year-olds that is worrying."
Mr Ashton may seek amendments, if the change is passed, to stop older men having sex with boys of 16, where the senior partner is in a position of "authority, influence or trust".
The MP believes that seven European countries have measures which protect young people from such risks.
Mr Ashton also said he was concerned that, when the age of consent for gays was reduced from 21 to 18 in 1994, it was done so on the basis of an amendment to a Bill passing through Parliament. There was no detailed analysis of the implications, just a three-hour debate and a vote, he said.
He was given assurances that the Home Secretary had ruled out "bouncing through" any measure to reduce the age of consent further.
Senior Tory MPs said last night that they expected the law to be changed to allow gay sex at 16.
"There was a small majority against under a Tory government. With Labour's majority, it will sail through," said one former minister opposed to the move. "We are allowing boys of 16 to indulge in gay sex but they may raise the legal age for smoking to 18. The mind boggles."
A Home Office spokeswoman stressed yesterday that, though the Home Secretary had said he would make provision for a free vote on reducing the age of consent, there was no clear timetable yet and no detailed discussion of the details of any legislation.