I've just been thinking about this again on my walk to work. Further to my previous post, when Grandad was waving his Pride flag in the 70s or 80s he wouldn't have been able to marry his husband, would likely have been subject to frequent, open homophobic abuse. If he was in Scotland he could have been jailed for having sex, up to 1982 I think. (Good ol' progressive Scotland).
Maybe he married Millie's Granny because it was the done thing and was only able to come out after she died or something. I find the possible back story all very sad. If it was real life I doubt if Millie finding the flag in the loft would make him think "Yay, lets have a fetish party"!
I find it pretty insulting to those people who had to actually fight to achieve the same rights rather than boo-hooing because they can't get everything their own way. I doubt if the young author and cohort actually understand how bad things were for LGB people not that long ago. Hence the attack on Fred Sargeant, an actual Stonewall rioter.