r10 is a cunt, but I must admit that some of the things he says crossed my mind. I, however, am no expert on socio-physical behavior in the 19th century, and therefore I cannot say for certain whether this type of closeness was normal among heterosexual men at the time. I bet r10 doesn't know it either, and therefore he should express his skepticism with more humility.
For example, about the photo in which the two lads are holding a paper saying, "Not married but willing", r10 says that they were merely "advertising" to girls, but what proof does he have of that? Was that photo sent by one of the guys to a newspaper? And if that was their purpose, to advertise to girls, why would they have chosen a phrase so ambiguous that it could easily be interpreted as a confession of love for each other and made them look ridiculous in people who saw it? "Girls, call us", "Girls, write us" would have been a much better message if that was what they were looking for.
The project's authors have seen hundreds of these photos, and they must have knowledge and arguments that neither he nor the others have here. I imagine that the selection was made according to some method and that the authors know from the context where the pictures were found that the men portrayed in the photos were not brothers or anything like that, as some are speculating here, and that they were not meant to be advertise the men in them to women.
Returning to the question of physical intimacy between men: I have some knowledge of the literature of that century, and I have already read a lot of biographies of my two favorite philosophers of the period, Schopenhauer and Nietzsche, and I know, based on what I read, that even these caustic men could write very warm letters to male friends. In fact, intense pathos can be seen in many letters sent by famous people of the period no matter the sex of the author and that of the recipient.
However, neither in these letters nor in the literature of the period is it possible to argue that physical proximity between men was commonplace, that sitting on each other's laps, holding hands, resting their heads together, was normal behavior among adult men.
Datalounge is, on average, one of the best educated forums on the internet, and I imagine that many people here know at least one classic of the time. Is this kind of intimacy between men seen in Wuthering Heights, for example? I read it more than once in my youth, and as far as I can remember, it is not. I know that in Moby Dick, there is a scene depicting two men sleeping together in bed, with their arms around each other. But this scene, by itself, does not indicate that this type of intimacy was common and, in fact, a critic of the time attacked the book precisely because of it, saying that the book “violated and defaced” “the most sacred associations of life ”(link below). Most likely, this kind of physical closeness between men had the same meaning at the time that it has today: that of passionate love, not brotherly friendship.
Another thing to consider is that, although photos were common from the second half of the 19th century, we do not have photos of any celebrities of the time in the positions shown in the photos. It's only anonymous men in these gay photos. And that, in my view, is an argument in favor of seeing them as homoerotic poem at the time. Because, if the whole thing was so innocent, I bet we would have many pictures of Nietzsche sitting on Richard Wagner's lap, of Nathan Hawthorne holding hands with Herman Melville, and so on - but there aren't.
I only know of one celebrity of the period who has pictures like that - Oscar Wilde -, which, of course, does not help the skeptical argument here. But the fact that, even then, such photos were compromising is illustrated by the fact that not even other gay celebrities of the period, such as Walt Whitman, dared to portray themselves in this type of position with other men.