If you're only wearing them when you work out or play sports it's completely fine. Never wear them in any other circumstance, though, unless it's just to wear to bed (if you wear a shirt to bed at all, that is).
A tank top on a guy is like underwear, it's not proper clothing, and they're meant to be worn underneath a shirt (unless it's a workout tank top), so it would be like walking around in public wearing just your boxer shorts. Sure, they're shorts and they cover your junk, but everyone knows they're not meant for that.
Maybe it's just me who thinks this way, but men who wear tank tops in public as shirts (not when working out) are like the trashy girls who practically walk around in their bras. It's very trailer.
If you're working out its to be expected and it allows for easier movement and its hot cause you have big muscles. However If you wear them on an every day basis people are going to think you're a show off and for some girls this screams tool.
Because you're in shape and you wear it while your working out, then its fine. I'll want to see those sweaty arm muscles. Sorry for being creepy ha ha :) I just wouldn't wear them as an all day everyday thing.