Poop, Crap, Sh!t

My wife and I were discussing this evening the arbitrariness of curse words. Or at least what is considered a curse word. Perhaps nothing is a better example of this than the various ways of saying "feces."

For instance, why is it that: almost no one gets offended when you say "my dog needs to poop"; some people get offended when you say "my dog needs to crap"; and a lot of people get offended when you say "my dog needs to shit"?

They all mean the same thing, yet one is always acceptable, one is sometimes acceptable, and one pretty much guarantees a fine by the FCC.

Discuss.

Also- why is it that you can't say "Fuck you" without severe repercussions, yet you can usually (on TV at least) get away with saying "Screw you." Again- they both mean the same thing.