Yeah that was what I was trying to articulate Queazy - you said it more clearly!
If YHA provide accommodation to both men & women within a given hostel, & they recognise a demand for mixed dorms (whether it's coming from mixed groups or just individuals who are happy with mixed sleeping), then I think it's fine for them to meet that need.
However, I think before they do so, they should provide single sex sleeping & invoke the necessary legal exceptions to KEEP it that way.
Then add a mixed dorm if that is what feedback is telling them enough of their customers would prefer.
The important thing should be that if a woman books single sex accommodation, that's what she is guaranteed on arrival.
If a mixed group is sleeping elsewhere in the building, having chosen & booked this, that should be OK too?