Some strong arguments against one-party consent:

1. Privacy. The cornerstone of personal conversation has always been an inherent right to privacy. Traditionally, conversations held in confidence were sacrosanct, and any disclosure of their content required mutual agreement. Otherwise, the second party can simply deny it and it will never be known with certainty what has been said.

2. Chilling effect on authenticity. In a climate where any conversation could be recorded without one's consent and later used against them, people would be compelled to adopt calculated diplomacy in all their interactions. This would sterilize authentic conversation, replacing genuine thoughts with measured communication, removing a lot of depth and meaning.

3. Weaponization of speech. Recordings under a one-party consent system could be misused in a myriad of damaging ways, from creating training data for voice (deepfake) AIs to insidious forms of blackmail. This extends beyond traditional notions of blackmail to include public shaming or character assassination. This is the "group chat has leaked" scenario, but potentially decades later. One could be a completely different (and potentially reformed) person, but their past will always haunt them and not let them get ahead in their public life, like hold an office or public trust.

4. Limited utility. In many places that allow first-party consent, these recordings are not recognized as valid evidence in court without consent from all parties involved. This significantly undermines their perceived utility as a defensive tool. Is it worth surrendering our privacy for this?

5. Accountability for human folly. People often vent and ramble, it's part of our emotional expression and a necessary component of our mental health. If every idle venting or thoughtless comment were recorded and held against us, it would make our lives much less peaceful. Sometimes it's good to let some thoughts die in a conversation. Reflect on your own past and the blunders of youth - good that they were not recorded, right? This also applies to intoxicated conversations, conversations while suffering from mental health illness, and similar.

6. Two-party consent already serves many purposes. For most business or evidence needs, two-party consent is enough. One can simply tell the other party that they will start recording the call if the other party does not hang up, and that will be implicit second-party consent in many countries. So one can record conversations, and one can choose to have only recorded conversations with some people. One-party consent has only marginal benefits over two-party, but it has significant downsides.

7. Little effect on harassment or threats. Threats and intimidation could be made in other ways, like face-to-face, or through coded language. "Your political campaign is going great, it sounds like a guaranteed win unless someone fabricates a scandal. You need to play it safe with the people that could."

8. Third-party recording. One-party consent doesn't mean a third party isn't recording. For example, smartphones could record audio of conversations for ad targeting and government purposes if there was one-party consent (one party turned on the call recorder).

9. Other contexts. If the laws for one-party consent are too permissive, this could lead to a lot of surveillance activities becoming permissible. For example, the definition of a conversation could be stretched to include all chatter on an office floor if just one person consents to the recording (perhaps a manager).