Anyone who disagrees with the Citizens United decision is dreadfully confused about what free speech means.
I respond that you are confused about the nature of a corporation, and that your opinion is not based on facts, however forcefully stated. The fact is that a few friends can start an organization with the goal of promoting their political views, and without the government telling them anything about what they may say. That organization is not a corporation.
A corporation in our legal system is a legal fiction, a tool designed to promote free enterprise usually by shielding the people involved from liability for the actions of the corporation. Thus, e.g. a small businessman doesn't lose his house and savings when his company craters. The government creates a corporation as a creature of the state by granting a charter, and that charter gives the corporation certain special rights and responsibilities compared to the hypothetical group of friends getting together, as above.
A corporation in our legal system -- and this is key to understanding objections to Citizens United -- has a legal identity separate and independent of the people involved. This is a useful concept in some ways. For example, in bankruptcy liquidation, a receiver may fire all the employees and disband the board of directors, but the corporation still exists in the eyes of the law.
People criticize the Citizens United ruling because it gives this independent legal entity some of the same rights as people, although it has many privileges granted by the government that people do not have. Crucially, the people in a corporation do not lose their individual free speech rights just because they are associated with the corporation. As individuals, they can speak out and give money to political campaigns. There are certain rules limiting the financing of political campaigns, ostensibly to protect the integrity of the democratic process. However, because it opens the door for the independent legal identity of a corporation to pour money into the political process, Citizens United gives the people who control that legal identity an easy end-run around the campaign-finance rules. Remember, the corporate officers still have all the rights to participate in the political process as individuals, but now they have a new, enormous avenue to participate in addition to their individual rights.
Perhaps you don't have a problem with that, but plenty of us do feel that it is wrong to inject the profit motive directly into the political process, in addition to the innumerable indirect ways it already (allegedly) harms people by distorting the process. That's a valid point that can be argued without dismissing your opponent's views categorically.