Hi. If I hadn't seen the previopus comments. I would have said "no way". Because co-ed is short form of co-educational which should be used referring to school and university classes or academic environments. But if native speakers use it, it doesn't matter what the origion is. There are many words created for specific purpose but they ended up being used for all purposes. For Suana
I agree with Hamid. In England we call them mixed saunas (male and female in the sauna at the same time/session). Sometimes it is "couples only" and sometimes mixed....and of course other times just men, or just women. I am not sure of the Am English equivalent....but I expect they are rare in the USA!!
You can rest assured that in British English it is "mixed" (I taught "English