Warning: long post
My guess is that you took a survey of who those protesters voted for in the last election, very few of them voted for Gore. Most of them probably voted for Nader, if at all. And there are a lot of "liberals" out there who pretty pissed off at them as well. So the term "liberal" can have a lot of different meanings. A lot of people on the more conservative side of the political spectrum use this term to lump together pro-military, free-market moderately liberal Democrats with the anti-American, anti-globalization quasi-Socialist Greens who are out there holding "Amerika is a terrorist state" signs. Usually when the word "liberal" is used in this way it's either to mislead or due to, for lack of a more polite word, willful ignorance. Anyway, what I'm trying to say is that there are a lot of people who would identify themselves as "liberal" in some sense who serve and have proudly served in the armed forces, and they deserve a little bit of respect.
When you take the commissioning oath, you pledge to serve your country and obey the orders of the commander in chief, not to serve a particular party. The proper question is whether you would be willing to make that commitment regardless of the political affilation of the president. For me the answer is yes. I get badgered on this point pretty consistently by my family (who are very far to the left and are pretty dead set against me joining the Marines), and I give pretty much the same answer every time. So, in a sense I would agree with what kn said earlier, that you should serve only if your political affiliation would not prevent you from carrying out your orders. However, that goes for both parties. You probably shouldn't join if you wouldn't be willing to carry out the orders of President George W. Bush, but you probably shouldn't join if you wouldn't have been willing to carry out the orders of President Bill Clinton either. The party that controls the White House has changed, on average, once every 10 years in the last century. If you've got a 10 year fixed-wing aviation contract, that means there's a pretty decent chance that you'll wind up taking orders from a CinC who doesn't share your political philosophy for part of that time.
That doesn't mean you have to like the CinC or think the same way he does, though. I think it's perfectly fine to express your opinion as long as you don't do it in an inappropriate place or in a way that interferes with your duty. As I've said before, internet discussion boards are about as far removed from actual combat as you can get, so this is a pretty innocuous place to sound off.
To recap: you serve your country, not your party. If you have a problem with that, you can do something else with your time other than serving in the military, like flaming people on internet discussion boards.
"Peace on earth to men of goodwill. All others stand by."