Most of my rides are, and I would like to be able to have it as a default, but I can’t! 

I don't think​ Strava has a way to set commute as the default, unless that's something which changed recently?

I would suggest checking your connected apps and see if any of them might be marking your activities as commutes.

https://www.strava.com/settings/apps

So, I figured it out. My rides were all showing as Commute in Strava.  This was a new thing as of a few months ago, so I thought it was a Strava issue.  It is not a Strava issue.  Activity Fix suggested checking connected apps. So I checked Garmin connect.  It’s not Garmin Connect.  It’s actually my Garmin 830, which records the ride, uploads to Garmin Connect, which uploads to Strava.  The setting in my Garmin 830 can be changed as follows:

“To set the type of ride on a Garmin Edge 830, navigate to the "Activity Profiles" setting, select a profile, and then choose the "Default Ride Type" option to specify whether your ride will be road cycling, mountain biking, commuting, or another category; you can also manually adjust the ride type after completing a ride if needed”

I had inadvertently had “commute” as ride type on my Garmin 830. Uncheck that and problem solved. No more Strava showing my rides as Commute and Strava saying how much Co2 I saved.

So, I figured it out. My rides were all showing as Commute in Strava.  This was a new thing as of a few months ago, so I thought it was a Strava issue.  It is not a Strava issue.  Activity Fix suggested checking connected apps. So I checked Garmin connect.  It’s not Garmin Connect.  It’s actually my Garmin 830, which records the ride, uploads to Garmin Connect, which uploads to Strava.

Thanks for figuring this out.   Was driving me crazy.   BTW - your fix works for a Garmin 1030 as well.