OK it looks a fairly standard letter asking me if I have acquired a partner and forgotten to tell them (which I haven't).
BUT there's a long list of ways in which you 'might be considered to be a couple' and one of them is 'You socialise as a couple/family'.
DS dad is not my partner. He doesn't live with me and DS, he has a home of his own, he doesn;t pay our bills, though he does give me money informally time to time (he is freelance) for DS, and looks after DS and stays here sometimes.
But we are not a couple. We do not date, or have sex, or behave towards each other in any romantic/sexual way; both of us occasionally date or have sex with other people...
I just find that definition of couplehood very unnerving.