The richest nonprofit site for showing male nude art over the centuries is called DelftBoys.com, put together by the wonderfully mad Dutch artist, Ad Shuring. To access its timeline of examples of gay art: 
go to http://www.delftboys.com; click “it’s all queer beyond here” 
(this will give you an overview page, which is worth checking out first)
then click “homo-erotic web museum” on the right side
then click the third item down, “lobby”
then click 4th item down “free art” 
then click “come on in”
  This gives you the “preview page of db fun”. Go down past the first four underlined artist names (the last is “Falco”) until you see the time line. 

Here is a link for going directly to the time line: 
http://www.delftboys.com/pre/fun/art7/artHIST.html
   This is one of those superblogs where the linkage is complex, bewildering, and fantastic. For example in addition to the history timeline DelftBoys has specialized galleries – but most of these like pre war Germany, history of model drawing, more model study, a huge collection of Nazi sculpture – plus Islam art had to be moved to the master class. level of viewership.
  This “masterclass” option offers premium level access to all the rest giving all images in hi-res for only $60 for 6 months: http://www.queerart.nl/mirror/pre/fun/subscribe.html  Click masterclass and you’ll see thumbnails of all images. Those with green links are free to access. Those with blue links require the paid options.
  It may be best to exhaust the free options – and then see if you want the full access. The amount of material here is very daunting. Suggest many visits vs burying yourself in it all at once. If you are a paying member this is the link for signing in: http://www.queerart.nl/mirror/real1/honour2.html
  Shuring has an utterly shocking wonderfully detailed account of his personal journey as a gay artist dealing with HIV in all this, the darkroomdiaries (free). He also offers reviews of many gay films. Keep in mind in dealing with all this that it’s the brainchild of one man. He personally deals with everything. You can even reach him live when he’s online. 
   LeslieLohman.org leads you to the first nonprofit gay art museum and foundation in the world, located in SOHO in New York City. Created out of a personal collection it has attracted donations of art works and sponsors and is in its third expansion, doubling its space on Wooster just above Canal Street: the re-opening in March 2017. Its openings and shows in the past have great merit. Its links section is overwhelming: it can be a rich resource for many hours of browsing. Membership not only gives you access to the edgier sectors of NYC’s sometimes shocking and delightful art scenes, it also gives you free entrance to a bunch of NYC’s museums – and a large number of museums across the United States.