I've no doubt you and hubby are perfectly decorous, as would be 'most' who wish to enter lounges in thongs, or similar. Unfortunately there are enough instances of gross behaviour (putting dirty feet up on tables, chairs; picking at said feet ... you only have to go back through this thread a bit; and there are others) for it to have been identified as a 'problem' (AKA lots of complaints).
I don't think the issue is thongs per se, but that they can be shed relatively easily, leaving bare feet.
So what to do? Order people out of the lounge if their personal habits aren't deemed up to scratch (embarrassing for both airline and individual and leads to confrontations) or create a rule to try to stop the issue arising. So a line is drawn. No matter where the line is drawn, there would be some who tried to test it, flaunt it or not give a stuff. Same with lounge RSA rules we discuss from time to time. Why shouldn't I be able to collect more than 1 drink at a time?? But the rule is there, so if I don't like it, I can go elsewhere.
I have posted a number of pics on AFF of totally gross feet on lounge tables etc - kids eat off the tables!! (not great, but we all know it happens). So sorry, to the thong etc wearers, the rule is necessary and should be enforced more stringently, IMHO.