Unlikely that only partial military uniform attire, like striped military trousers, would have been worn in a clearly civilian social gathering like in this photograph. In my view, it would not have been correct etiquette at the time (a full dress uniform would be okay though).
More likely that the striped trousers were part of a socially worn black tie suit (or possibly even a tuxedo). This type of dressware originated around the time of the Civil War.