Great picture of men dancing together from the 1904 in the river. This comes from the Archivo General de la Nación? in Buenos Aires.
There are actually many theories as to why men danced together in the early days of Tango. I think there is some truth in all of them.
1. This was a time when women were not out in public as much as men. Women were in the house and if they went out in public or to public events then they were accompanied by a male relative. Men learned to dance with one another on street corners, clubs and as this picture shows anywhere they could gather.
Women learned primarily in their homes from their brothers, fathers or mothers. So, men would HAVE practice with one another unless they had a female relative to practice with and women were few and far between. Most Buenos Aires was one of the busiest cities in the world and was enjoying mass immigration from Europe, but this was mostly men coming for jobs while their families stayed back home.
2. There were simply not many women to dance with, so men HAD to practice in order to get good enough to dance with the few women that were around. Some estimates state that men outnumbered women 7 to 1, at the turn of the century. This was also still a time when Tango was considered a lower class activity, so many women would have refused to dance this obscene dance.
3. It is also thought that perhaps the early, early Tango was a primarily male dance. That its early origins were on the outskirts by Argentine Cowboys gauchos who would dance around their campfires competing with one another doing very athletic movements and that tango became more tame as women became more and more involved.