You're a grown up. If you're commuting on a crowded subway and Justin Timberlake happens to come up on the shuffle, well, so what? You don't have to scramble to lower the volume or skip to the next song because you're comfortable enough to enjoy catchy, meaningless, pop music without any sense of shame.
Or it's because you finally broke down and bought a pair of earphones that didn't come free with your ipod, like the Shure E3c earphones. Not only do they sound better than those tinny Apple earbuds, they block outside noise. Yes, this leaves you vulnerable to being stabbed, run over, kidnapped by a gang of hobos, etc., but your ears will thank you. No need to crank up the volume anymore because now you can actually hear the music. More importantly, noise-blocking technology means your music no longer pollutes the eardrums of your fellow travelers. That guy standing next to you on the train has no idea what you're listening to until you pull out your ipod and Hall and Oates are right there to let him know that you mean business.
There are plenty of earphone options out there, depending on how much you want to spend, but buy something for God's sake. It's time you kept your music to yourself.