'Experimentation' (if you call it that) really is a lot more normal than most people would have you believe - I won't make assertions about majorities, but seriously a -lot- of straight men at the very least look at guys even if only once or twice (and yes, some of them go on and do stuff with them).
Though I'm not at all suggesting you're bi, for what it's worth I have been told by several bi friends that they 'alternate' - they go through times when they like one gender, then another. I'm not suggesting that's what you're doing, but it does indicate that it does sometimes happen that you start to like the same sex for a period of time.
Do you want to stop looking at men? There's not necessarily any harm in it, and it does sound like it's just 'happened' - to be honest I really don't think there's much you can do to stop yourself looking - sexual attraction is just about impossible to control (though how you react to it isn't). It all depends on what you want, really. Hope that helps.