The SOP term does denote that one Penguin more significant than other Penguins I might be dating - and that has indeed been the case. When I have chosen to date additional Penguins, it is usually under the full knowledge that the SOP is my primary partner. If the other Penguin would like to become my SOP, then that could be negotiated but I feel (after 11 years) that it would be very unlikely.

It is difficult to date two Penguins at the same time - but a 'relationship' is simply that, it denotes the rules you have between you. You are in multiple relationships with everyone - now you're in my comments section, you're in a relationship with me. How you apportion your time is up to you - and is largely about your own management style. If you manage to hold down multiple friendships, then you already hold down multiple relationships, it's simply about what you consider to be included in your time. :o)