Hi Jonathan,
that's very unfortunate. We are not yet migrated to ArcGIS Pro, so we will have to stay on ArcMap for a while. As an admin I can switch between portals via the ArcGIS administrator, but all the ArcMap users of the organization can not. For them, there is no way to connect to Portal and be able to browse the ArcGIS Online contents. You are positive that there is no such option? Or via a workaround by connecting to some ArcGIS Server or something?
The problem is that some of my users want to be connected to our Enterprise environment, but others will not use it. I can only set 1 default in the ArcGIS Administrator application for all users, which makes it a difficult choise to make.