How can I quit Steam on macOS 10.12 without force quitting?
I had this same issue. I lodged a ticket with steam support and they told me to enable beta client.

Here is a link about how to do just that that 

After signing up for beta it is now quitting for me and my double quits are gone. If you don't want to participate in the beta, hold on to your butts; a fix is probably coming soon when they get a stable beta.

-- I did say soon in my initial comment back on Apr 5 '17 so perhaps not so soon if you are here now.