There are a lot of relationships one can be in. You're in a mutually amicable relationship with your friends, in a familial or parent–child one with your parents, in a professional or business one with your co-workers and in an on top–below one with a table you are stood on top of. However, if you were to say 'I am in a relationship with my mother' or 'I am in a relationship with my co-worker', people would assume you to be partial to incest or consummately unprofessional, respectively.
When did the default assumption for the nature of 'a relationship' become 'a romantic relationship'? Has this always been the case, or is it a relatively recent phenomenon? If the latter, can we blame it on Facebook?