Can't see the problem if couples want to use FMF, it's not illegal is it.
Privacy within a relationship is between consenting adults, shaming people for being entirely open is quite patronising, many mariages police their own lives and rightly so.
Just because some people do not want these apps to become normalised for fear of appearing untrustworthy is not other people's problem. Certain people don't like a timestamp on Whatsapp, but rightly or wrongly it has become a sign of someone being, not as open as they could be. People judge on how open and transparent people are, that's how it is.
Protestations that are pointless, if you don't want to be on the app, then don't be, but don't expect to feel superior to others who use it, many use it not for nefarious reasons but for safety and love within a family.
Totally normal, and it's going to become even more normal, whether you like it or not, it is being woven into todays society as a natural progression of life, in 20/30 years time it may even be required as pre requisite for eliminating dodgy males/females to settle down with.
The key word here is choice.
It's the shaming that gets me, making people feel like they are acting criminally for knowing where their other halves are.
I live alone and I would love for someone to know where I am. 
Sp op, bugger the pious privacy brigade, it's your relationship, and yes your husbands been an arse (why was he on his own ?) but I suppose everyone needs a blow out now and again.
One to tell the grandkids.