I have always found it curious that some people will advocate government spending in a particular area, yet they would never spend any of their own personal money to fund it. While it doesn't seem to make any sense, I can see some rationale behind it.
For instance, it would be an interesting survey to ask 100 people if they think that NASA should be abolished. My guess is that a good majority would say no. Then ask them if they are interested in contributing money to a charity that is involved in space study and sending people and satellites into orbit. My guess is that a majority would not contribute anything out of their own pocket. Perhaps some might donate a few bucks. There might even be a few who donate hundreds of dollars, thinking it is a worthy cause with a lot of potential. But it is rather obvious that the responses to the two questions would not match in a lot of cases.
So why would some people want to fund NASA through the government, yet they would never spend a personal dime on such a thing? There are different possible explanations.
Some people don't want to pay for something when others are not. Someone might want to contribute his own money to NASA, but he also wants everyone else to contribute too. The only way to get everyone to contribute a little is by using the force of government. People like this don't want any "freeloaders", even though they are the ones who are advocating force.
Ironically, we live with freeloaders our whole lives. You are probably a freeloader yourself. You don't have to shop at Ebay or at Walmart, yet you probably benefit from these entities in that they provide competition and get other stores to lower their prices.
You could also be a freeloader if you walk into a mall or a store and browse and don't buy anything. You might even enjoy some of the Christmas decorations or something else visually pleasing, yet you haven't paid anything.
Ironically, it is usually the rich who pay more, while the freeloaders tend to be the lower and middle classes. When a new technological gadget comes on the market, rich people will pay thousands of dollars to be the first ones to have it. They are the guinea pigs. The prices eventually come down. Now we can buy a big screen television for under a thousand dollars.
Another reason someone may advocate a government program, yet never pay for it voluntarily, is that he sees his tax money as a set number. If you asked people for a choice between getting rid of NASA or paying twenty five dollars out of their wallet on the spot, then a lot more people would want to get rid of NASA. Most people, and probably correctly so, realize that NASA could be defunded, yet they probably wouldn't see any of the extra money. From this standpoint, taking differing positions might actually make a little sense.
I think a third reason for this seeming contradiction is that many people simply do not think about it enough. Some people assume that if government didn't fund NASA, that all space exploration would come to an end. Libertarians should be able to relate to this, because we witness a lot of lazy thinking. As a libertarian, it is common to be accused of being in favor of drugs, just because I favor drug legalization. It is common to be accused of being against education, just because I don't favor government run schools. It is common to be accused of being unsympathetic towards the poor, just because I don't think anyone should be forced to "give".
I think this contradiction of some people advocating government spending on a program that they would never voluntarily fund is a good example to point out to others. It is a good technique to have a conversation about libertarianism, without making it into an argument. You don't even have to take a position upfront. You can just say that you find it interesting that many people favor war, yet they would be unlikely to voluntarily donate money to the government to fund going to war. You can use this for any government program that is up for discussion. And even if someone were willing to voluntarily fund a particular program, does it mean that everyone should be forced to fund it?