This argument seems a successful defeater to me (not that there haven't been other ones) of Thompson's violinist argument for abortion:
One of the most sophisticated case for pro-choice comes in the form of what Dr. Judith Jarvis Thompson called the violinist argument. Briefly, this argument goes like this: Say a world-famous violinist developed a fatal kidney ailment and the Society of Music Lovers found that only you had the right blood-type to help. So, they therefore have you kidnapped and then attach you to the violinist’s circulatory system so that your kidneys can be used to extract the poison from his. To unplug yourself from the violinist would be to kill him; therefore, pro-lifers would say a person has to stay attached against her will to the violinist for 9 months. Thompson says that it would be morally virtuous to stay plugged-in. But she asks, “Do you have to?” She appeals to our intuitions and answers, “No.”
The strength of this argument is that it grants the full humanity of the fetus and therefore sidesteps the rather poor arguments we saw above. The weakness of this argument is that it is an argument from analogy. Arguments from analogy destruct if they rest on fatal disanalogies.
Besides many of the other problemspro-lifers have pointed out, here's a big one. Thompson seems to make a distinction between consent to pregnancy and consent to sex (as Beckwith and others point out). But it seems that pregnancy is the designed result of sex, even though it may not be the desired result. It would seem that our sex organs have the purpose of being ordered towards procreation. Applying this to the violinist then: What if I engaged in an activity, say, spelunking, that regularly created rare kidney diseases in violinists? Say that every time I dropped 50 ft into the cave, a violinist was almost sure to develop the disease that only I had the blood type to correct or fix. If I did so, should I not be hooked up to him, voluntarily or not? Say that there was protection, some kind of spelunking helmet. Say that it was not 100% effective. If my helmet ripped, should I be attached to the violinist? Or say I tried to "pull up" before I hit 50 ft. Unfortunately, it felt so good to decend that I pulled up a little too late and my right foot passed the 50 ft mark. Should I be attached to the violinist? I don’t see why not. Indeed, say that the statistical evidence was that the first two people that ever spelunked together would eventually cause 6 billion violinists to come down with rare kidney diseases, I dare say the Society of Music Lovers, and almost everyone else for that matter, would call for abstaining from spelunking unless you agreed to take care of the violinists until they got better. This seems fatal to Thompson’s argument.