There's something strangely erotic about a dude in a singlet - I mean, you know what I'm talking about, yeah? When a dude's sweaty, or covered in muscles and bulging hella hard after a massive session at the gym. Well if you're pretty in tune with what I'm talking about, you might want to check out /r/Singlets. The name says it all, and with over 4,500 subscribers, it's pretty damn populated with people that want to see these hot studs getting sporty in their singlets.
There are a few self-posts on /r/Singlets, although I should mention that not all of the material is gay. There are some straight bros, but as far as I know, most of the community doesn't mind that so much!
For a sense of how regularly updates come in, /r/Singlets has had 10 posts in the last week. It's a little low, but hopefully with some exposure, it can become a top quality subreddit where we can all enjoy dozens of singlet-clad guys that we'd pay top dollar to fuck.