I've thought of two animal related ones:
I hope you didn't find that ...unBEARable. (the story somehow involved bears)
I had a WHALE of a time. (the were shagging on a whale)
( brb Monkhouse Stampede, Thu 24 Jan 2013, 0:09, archived)

oh DEER
he loses his erection after they are startled by a deer
( 2 Can Chunder Word to your mums, I came to prod bums, Thu 24 Jan 2013, 0:14, archived)

She could have a throat infection and he says 'you sound a little HORSE' but also they are doing it on a horse.

( 2 Can Chunder Word to your mums, I came to prod bums, Thu 24 Jan 2013, 0:18, archived)

He goes 'you seem like a nice BIRD', do you fancy sucking my COCK or are you CHICKEN cos it tastes UNPLEASANT (pheasant)

( 2 Can Chunder Word to your mums, I came to prod bums, Thu 24 Jan 2013, 0:24, archived)

"DAS BOOTS"
They were doing it in Boots
( 2 Can Chunder Word to your mums, I came to prod bums, Thu 24 Jan 2013, 0:29, archived)

You seem a little crabby
They're doing it on a beach but also she has crabs
( 2 Can Chunder Word to your mums, I came to prod bums, Thu 24 Jan 2013, 0:32, archived)