Curiously enough a few of us were discussing this sort of thing in the pub the other friday.
Serious question - if you had to with a farm animal, which type and why? My own thoughts on the matter are as follows. I would add these are simply observations and do not construe scientific advice:
Donkey - see previous mental imagery above...
Horse - too massive and smelly. Nice arse, though - for an animal.
Cow - repugnant slathering buggers with awful sh*t-covered boney arses. However they do smell of leather if you like that kind of thing.
Pig - if it was a nice chubby pink one, that could be nice. But they've horrible faces. And that curly tail would put you off a bit.
Chicken - probably a bit small, but that bloke used to shag them in that German "Animal" film. But they'd probably try to peck your knackers - in a bad way.
Sheep - smelly, but soft. Like humping a blanket.
I think for me, sadly, that the answer would have to be the rather obvious sheep.
Obviously this is only if I had to. Like if they were going to kill my mum unless I wandered into a farm and shagged one of the beasts. Not, like, for fun you understand.