Apple - make airpods microphones disabled by default when pairing with a mac
Ok, here is a script that would select Internal Microphone..
Copy Paste following in the Script Editor in Utility folder.
Run it, if you like it we can make it a keyboard short cut.
tell application "System Preferences" to activate
tell application "System Preferences"
reveal anchor "input" of pane id "com.apple.preference.sound"
end tell
tell application "System Events" to tell process "System Preferences"
tell table 1 of scroll area 1 of tab group 1 of window 1
delay 10
select row 1
tell application "System Preferences" to quit
end tell
end tell
If you're willing to spend $5, get ToothFairy.
Not only does it make it much easier to pair with your Mac, but it has a feature to do exactly what you're asking for under Settings » Advanced:
Check that box at the top and voilà!
UPDATE: ToothFairy is now also part of Setapp.