Unless you're using an old fashioned wash board and scrubbing everything by hand, I don't see how it's any great hardship to just turn the bloody washing machine on regardless of whose clothes are in it

I agree. I think everyone doing their own washing is very inefficient and not very environmentally friendly. Both DD and I own a lot of white/light coloured clothes, so they get separated out. None of us own enough clothes to make up a full load of one type, so it seems bonkers to me to make everyone do their own.
Do the households that make everyone do their own washing not even wash things like bedding and towels together?
I have neither the time or inclination to separate mine my husband and babies clothes into separate washes. It's darks and lights here and that's as far as it goes.
And that is how our household operates. I agree with the comments on the parallel MN universe because I don’t know anyone in RL who does this.
Possibly some posters dont have teens/adult DC living at home
DD is 17 and up to her eyes in A level revision. This thread has prompted a discussion about her needing to know about everything required to look after herself at university – including how to use a washing machine. And before anyone berates me for not making her do the laundry/clean the house from top to bottom, wash the cars, go to work before she goes to bed etc, she is on meds for anxiety and depression, and I am not prepared to add to her worries just now.