There are several threads on this subject so a search would really help you with lots of ideas. However, I had (nearly) that exact experience recently. Only difference is I felt I was nearing the point of rounding the bolt so thought I should go to plan B before I ended up in that situation. As far as why it is so tight, I can give you a possibility. On mine, it had never been removed in 7 years and 140K miles. The dealer had always used a topsider. So the copper washer had gotten kind of stuck to the plug (my opinion). I had a good 6 point socket on it and was using A LOT of force and it seemed as if it was welded on. One issue is that I had "warmed" up the car (more like hot) and so I suspect this made the situation worse. What I did then is I sprayed the bolt head with "freeze-it" spray until it was cold on the theory that this would shrink it a little (I am a dreamer).. Anyway, it worked and I was able to break it free with no damage.

But you have a rounded off bolt head. Again, I would really suggest searching, but one of the ideas I remember is these special sockets Sears sells for just such an occasion. And also try when the engine is cold. And when you put the new one on, a little never-seize won't hurt!

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra