"Lair of the White Worm" (1988)
A boy scout gets his penis bitten off--or, at least, bitten--it's hard to tell, when the villainess in the film goes down on him, while he is in the tub.
That movie is hilarious - Hugh Grant at his lowest point! The one thing that really puts bad movies over the top is when they involve actors that went on to do great / noteworthy things, and you can just imagine if you ever met them in person, shook their hand and said "I loved you in Lair of the White Worm".
And the worm chick was played by Jim Carrey's boss from LIAR LIAR too, so she's probably not too proud of it either.
If you mean amanda donohoe, she was in a movie that made LotWW look like Gone with the Wind.
It was starship stormtroopers 3. Good god that movie was rancid.
BTW, if you love "knee to the groin" scenes, you need to check out a classic Dr. Who episode, can't recall the title but it had the master and the rani in it, and when the master tries to take contorl of the rani's tardis she knees him in the groin and I swear Anthony Ainely, who played the master, deserved an award for that scene....