As PF notes, the whole story (such as seperate PNR's perhaps) is not known, however if it were me, and a same sex partner and our (young) child in a 2-2 config, I also wouldn't be moving just so a 70 year old couple could sit together. They've most likely had many years together, while my hypothetically young child hasn't had many years with us. Flying can be scary, and I'd want to be in eyesight of them at all times to reassure them everything was okay.
Besides, there would have been 6 sets of 2 seats in a 737. Are you telling me everyone onboard was at least a member of a couple that flight and the best family unit to displace was the one with a young child? That's rhetorical.
While this may not be homophobia, it's at the very least rude if the CSM made a fuss of the situation and then didn't apologise when realising their mistake. Turning and walking away isn't the standard I'd expect on QF.