Hmmm ... 
I was going to start my own thread but I think it's more relevant to post here?
I was in M and S yesterday picking up some outfits for tween DD. Very near store closing time and no staff on the floor I could see. Could only see one fitting room which was next to the men's suit section. I jumped in with DD. All cubicles with locks with walls from the floor going up high - but not touching the ceiling.
This was not labelled a fitting room for any particular sex but it was very clearly intended for men as there were only men clothes on the 'return rail' thingy as you walked in next to the desk. There was a man and his wife in there (wife sitting outside the cubicle, running backwards and forwards with different shirts). The wife stopped me and DD as we walked in and said something along the lines of, "Oops, this is the men's changing room." I explained we were in a hurry and couldn't find the children's fitting room and the other one was all the way downstairs. She didn't say anything else at that time but I could hear her and her husband mumbling a bit about it. From the man's expression (he came out every now and then to show his wife his shirts), he did look quite uncomfortable with me and DD being there (though his wife was sitting outside too! I stayed mostly in the cubicle with DD but occasionally went in and out to get different sizes). DD really didn't care at all as long as the door shut and was locked (she hates fitting rooms with curtains regardless of whether or not they're female only).
When it got to paying, the woman had obviously said something to the shop assistant as she told me where the children's fitting room was 'next time I visited'. I explained that we were in a hurry as it was near closing time and I hadn't been able to find any staff at the time to ask so it just made sense to go in the nearest fitting room.
Now, I actually preferred the fact I could go in the first fitting room I could find. I also liked the fact that the husband's wife felt able to sit outside his cubicle to help him out with getting different sizes etc. We felt safe as the door locked and he walls were very high. My experience yesterday completely changed my outlook on mixed sex fitting rooms. I only saw the benefits of them yesterday. It was much quicker and easier than finding one meant for a particular sex or age.
If cubicles can be locked and there aren't large gaps in the walls, I would actually prefer fitting rooms to be mixed sex. I think though there may be some inconsistency with the staffs' thoughts towards the fitting rooms such as the lady yesterday trying to direct me to the much further away children's fitting room.
I'm sorry if this is the wrong place to post my experience but I thought it might be ?interesting? to hear of a gender critical person's thoughts of the benefits of mixed sex (and age!) fitting rooms.
I later asked DD if she would have still felt as at ease if I hadn't been there with her. She said yes as long as there was a high door with a lock. Whenever it's a curtain, she always makes me stands outside to hold the curtain in place even when it's female only fitting rooms.