If you take the green movement on their slippery slope you can reject material comforts, you can get rid of your SUV, then your car, then downsize your house, stop flying, even stop purchasing anything not grown locally. You can essentially lead the constricted life of a midieval peasant.
But why stop there, you can continue on restricting yourself to two, then one, then zero children. Perhaps even get to hoping, or acting on, a desire to cull the human race of a few billion polluters. This will mostly annoy people but we can handle all that.
But, when you want to take my dog, you have crossed the line. So a dog pollutes more than an SUV. Man's best friend indeed.
We wonder whether result of the relentless and radical expansion of eco sins will be to hasten the movement's collapse into self-parody, or make less extreme hair shirts seem more acceptable — Sure I'll recycle, but I'm not going to kill my cat.