Recently, philosopher Michael Sandel headed this debate (essays, public lecture) on the moral dynamics that result when money is introduced to activities that are normally motivated by other means: duty, friendship, etc. Sandel makes an argument of two pillars: One, money may somehow corrupt the transaction it is involved with; two, given existing inequality, making everything a product can yield very negative distributional outcomes.
To be honest, I think he uses too many words. Let me try to do better:
1. Subjective transactions
Sometimes, we care not just about the objective goods or services that are produced and exchanged, but also for the subjective experience of the participants in the transaction. We care that a gift is an expression of love and not a forced or calculated gesture. We care that expert or legal judgements are free of personal interest, and that honour is given according to merit rather than in exchange for favours. Men care that women are at least happy with sex, unless they’re sadists and want the opposite. Most of us are alarmed when others make transactions that are too invasive or too damaging to the self, such as selling organs, being a soldier or a prostitute, or working under hazardous or unhealthy conditions, because we suspect these people are facing terrible choices that we would like to not have to face. When people volunteer to do something worthy or refrain from something selfish out of a sense of community, duty, or justice, we care not just that they do the right thing but also that they maintain and nourish these valuable feelings.