I haven't looked at night trains in a while, since CNL stopped running them, but, as I remember, only "sleepers" were gender specific. Since in a 'couchette' you were expected to sleep in your clothes, genders could be mixed. If a mixed gender couple wanted to book a sleeper, it had to be a two person sleeper. The only way a mixed gender group could book a sleeper, was if they booked all the beds in the sleeper.
Do a dummy booking for the 21:06 NightJet from Berlin to Baden-Baden. If you request a couchette, it won't ask you your gender. If you try to book one person in a sleeper, it will ask for your gender and say, "Due to customer demand, multi-bed compartments for single persons are only offered as single sex."