Lord Ponsonby will announce plans on Monday in the Lords as the Bill - which would introduce tougher penalties for juveniles and tighter bail conditions for defendants - has its Second Reading.
His age of consent amendment would clear up legal anomalies caused by MPs' decision to lower the homosexual age of consent from 21 to 18. Under the Bill as it stands, boys of 16 and 17 face prosecution for homosexual intercourse, even though underage girls cannot be prosecuted for having heterosexual sex.
The measure would mean in both cases only a partner aged 18 or over could be held legally liable and taken to court.
Lord Ponsonby said: 'I want this debate to focus on the anomalies of the age of consent rather than reopening the issue of the age itself.
'The whole argument for 18 rather than 16 was to better protect young homosexual men. Criminalising them doesn't seem a very good way of going about it.'
He believes a new offence of male rape would also resolve anomalies. Under the Sexual Offences Act 1956 the maximum penalty for rape of a woman is life, while male rape is not recognised. The charge in such cases is 'buggery without consent', which carries a 10-year maximum penalty.
Both amendments are likely to be close-run. The chances of decriminalising men under 18, in particular, are said to be too close to call, although Stonewall, the gay lobby group, believes many right-wing MPs would like to prevent underage prosecutions. 'Few MPs who voted for 18 really want 16- and 17-year-olds arrested,' Roger Goode of Stonewall said.
Lord Ponsonby is keen to table the amendments on Monday to overcome any Government objections. But he might have to delay if another amendment emerges to restore the homosexual age of consent to 21. In that event a counter- measure to lower it to 16 could be tabled and Lord Ponsonby's measures put on hold until the Bill reaches its Report stage at the end of the summer.
The flurry of movement follows the defeat of the passionately fought campaign for a homosexual age of consent at 16, voted down in February. MPs voted to reduce the age to 18, provoking scenes of violence outside Commons where homosexual men held a vigil.
It was feared then that the defeat of the bid to equalise the ages of consent would lead to stricter enforcement of the law by police and the Crown Prosecution Service and a rash of arrests of 16- and 17-year-old gays. But no action is known to have been taken against them or campaigners such as Euan Sutherland, 16, of Dulwich, south-east London, who repeatedly appeared in the media calling for the right to have legal sex with his boyfriends.