Can I keep "Leave game as group" checked by default?
Solution 1:
This has been added in the November 16, 2017 patch.
Clicking the "leave game as group" checkbox will now remember your selection for future matches
Solution 2:
Update: As Wrigglenite pointed out in their answer, this feature has now been implemented.
At the moment, it doesn't seem to be possible (I checked every options page multiple times) but you may suggest that feature in the Overwatch Forums.