The young couple in the previous post are Clara Barton and John J. Elwell, the young man she was linked to romantically.
While there is no evidence that they were ever sexually intimate, some time after the war, General Elwell wrote to her that he loved her “all the law allows (and a little more perhaps)”. What exactly that meant remains the realm of speculation, as it is certainly cryptic by intent; General Elwell was a married man. Given that he was married at the time of his association with Clara Barton, this photograph becomes an act of bravery and defiance (or brazenness depending on your take of Victorian social mores), although perhaps it could have been passed off as innocent as Ms. Barton and General Elwell both served together in South Carolina at the assault on Fort Wagner, he with the Quartermaster’s Corps, she as a civilian nurse. Certainly at the time she was already famous, and he would have been so as well by the time the photo was taken in 1865, so it may have been at Mathew Brady’s urging that they posed together or it may have been of their own choosing.
Here is an image of General Elwell:
and Clara Barton:
And my photo for comparison: