I think the "green" argument against beef/pork/sheep/etc. is typically around the amount of fresh water expended, methane produced, and waste generated (both while they are alive and after).
I don't really think cow farts are going to do us in as a species. The only part of the lot that might have a little merit in my opinion is the amount of water required to raise and feed the animals. On a stretch, you could also argue that cattle (more so pigs) do a number on the land. Ever put a bunch of pigs in hilly country? Makes for a mess. It's amazing how you go through many parts of the country (or the world for that matter) and vast forests are reduced to grass, clover, and dust covered pastures.
Generally speaking, I figure most deer live in the forest and forage. The are very lean (and hence good for those of us with fat around the middle). They don't really cause destruction of the land and we don't clear pastures to put ****loads of them in a small area. If we did...I'm sure the beef debate would include deer (reindeer are already farmed for meet, eh?).
U.S. Beef tastes GOOD...and so was my great granny's fudge (God bless her soul). But too much of either one ain't good for staying upright in the world.
So...in conclusion....cow farts ain't gonna kill us...wild deer are a great alternative meat source while they last, and cows take a lot of land/feed/water/etc. to make one pound of protein compared to other sources. We're American, though....who gives a crap about all of that...we eat what tastes good!
I SOO miss the taste of a nice juicy rare USDA prime steak. I find the beef over here inedible...and this thread has made me hungry!