At M&S recently with teen for bra fitting.
The cubicles are double ones, so a locked full length solid door with another curtained area inside.
I heard the staff turn a man away who was wanting to try something on and directed him to other changing.
He said there weren't any signs to say it was just for women and he was told that as it was in the lingerie section and fittings taking place there may be women in who may be uncomfortable.
He went off, but even assuming no dodgy intent on his part, whatever direction he approached from he had walked through several aisles of lingerie, so how he could have thought it was appropriate I really don't know.
Credit to the staff for stopping him though.