Ok, so I get that Triathlons are boring, and I get that training for tri's is also boring. That DOES NOT mean you should shut out the outside world while you do hot laps up and down the bike path. Commuters: same shit. I know that you don't know how to ride a bike anyway, and that's okay, I'm happy that you're swerving down the bike path instead of swerving into me in your SUV, but do you really need to complicate something you apparently struggle with already by removing one of your senses?
Road cyclists, you have no excuse. One of the purposes of getting on the bike is to escape from the constant drone of technology. The other is to suffer. Of course there is also an implied goal in there to make it home without getting run over. Listening to music inhibits all of these. You might as well be driving your car or sitting in front of your computer at work. And the suffering: music distracts from it. You really shouldn't short change the suffering.
I'm not buying your "it's not very loud" or "it's only one ear" bullshit. Even turned off completely, earphones block a fair amount of outside noise, and I have no desire to make myself blind in only one eye. When you are cycling, it's best to assume that every single other person is both completely unaware of your presence, meanwhile they are trying to kill you. If you were being hunted by ninjas in big lumbering SUV's, would you not want to hear them coming, and at least brace for impact?
And then of course there's me. I have to somehow get around your path swerving bumbling beach cruiser, road bike with clip ons, or whatever. Trying to predict when to pass is a lot like watching plinko, and it's not made any easier when my "on your left" falls on deaf ears.
To sum up, just don't do it. Of course if a tree falls in the woods and nobody is there to run you over, go for it, but if it's in a place where me, or anybody else can tell that you're trying to block us out, we're all going to think you're a douchebag.