I really wish people would stop using this erroneous argument. "There's
fireballs in D&D, so
nothing has to make sense, and reality is impossible!"
Bull-dookie (yes, I used the word dookie, out of respect to Eric's Grandmother.

) Just because one aspect of a campaign is fantastical does not mean you check your sense of realism at the door. Basic gravity, physics, etc. still apply. And, IMC, the general behavior of farm animals. I only change the reality I want to, and I see no reason to suddenly make pigs behave other than naturally, in order to invite Cheese into my campaign.
Granted, as someone else mentioned, they could simply switch to sheep. At which point I might get a chance to pit the PCs against some of the good celestials I've always wanted to try. Those sheep have a god too, and that god has servants. Sacrificing sheep in such a callous way is bound to piss somebody off Upstairs.