Wikipedia agrees:

Head and Fisher never married owing to her indifference to marriage; however, Head considered the pair to be a married couple.

I agree that it's weird to call people in their 60s and 70s who aren't married "boyfriend and girlfriend". But that's because it's weird to be in your 60s and 70s, having lived together for more than forty years, and not be married. The answer isn't to use the term 'partner', the answer is to just get married, u/Cyberguy's uncle notwithstanding.