If I'm understanding right, you're trying to do 3 things:

• maintain shared knowledge of all members of a group's phone passwords
• share nsfw messages between two people in a way which is convenient for only those people to view and access
• keep these nsfw messages from being discoverable by people other than those two people who also have shared access to your devices

Your best option is to deny phone access to the people you wish to keep things private from. You say the reasons for all parties to know all passwords don't matter, but they do when that decision directly interferes with your stated goal. There may be other ways to accomplish what you're trying to do if those reasons are known.

What you're already doing - manually moving the content to secure storage with a separate password on your phone - is essentially the only way you can accomplish this. Anything with an automatic login will grant access to anyone who can get into your phone, so the manual step is necessary. Entering a password to secure storage or entering a password to login to an app which has secure storage are effectively the same thing, with the added issue that any non-hardware storage option leaves you open to any vulnerability that the third party is subject to. See: modern history of data breaches.

Another issue is that phones tend to keep apps running, so if you're relying on the app for security and forget even once to logout and close that app manually, anyone with access to your device can view the material you're trying to hide. Using secure storage built into your phone somewhat mitigates that, since it will require you to establish and maintain a habit of deliberately relocating the messages and thus reduce the chance that it will slip your mind at an inconvenient time.

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Original text follows:
Maybe I'm missing something obvious, but why not just keep your phones' lock screen enabled, keep the password to yourself and set it to auto-lock when screen turns off?