"Do you think it would be OK for a man to use the female changing rooms in a store when buying (say) new underwear. Seems a fairly close parallel."
I'd probably be inclined to leave that one down to individual store policies. Or, perhaps (because, unlike in a toilet, one can always try these things on at home), if someone complained about it then I think it wouldn't be unreasonable for the person in question to accept the complaints and either come back another time or go elsewhere to try their clothes on.