The mixte frame appeared in the years following WW1. Youll have to go back a lot further than that example to discover its origins and marketing.
Indeed, the very word mixte harks back to its origin as a mixed sex frame.
What happened in the years leading up to WW2 and beyond where it did indeed become feminised as ladies frame has nothing to do with its origins.
What it became was not what it was intended as at birth.
And now things have gone full circle, and boutique mixed frames are again being sold as suitable for both, again, with renewed marketing emphasis on the reasoning behind the origin of the name mixte.
Considering most people buy off the shelf frames anyway and very few give any genuine thought to detail fit, there is no reason why a mixte frame is any more or less suitable for anyone provided it is sized correctly. There are plenty of blokes out there on sub optimal fit mens frames that seem happy enough with their lot, just as plenty of fellers drive Fiat 500s.