As PP have said it’s about containing my droplets and minimising risk of breathing them onto someone else. So as I share saliva, breath, etc with DP I don’t think I’m at risk from using even one he’s had on briefly and as long as it’s clean that day, and not saturated with moisture, it’s not a risk to others.
You haven't quite understood.
I agree that the likelihood that any of us are handling masks fully correctly is small.
Ideally, you carefully remove & store the mask in a proper storage bag (for example), ensuring it doesn't come into contact with surfaces,hands etc. As technically, this would allow the virus to transmit.
I get that we aren't doing all this. But sharing it with someone else is increasing the risk of transmission.
But also, yeah I think it's gross!!