thank you for understanding the spirit in which the observation was made.
As a guy in an urban area who crosses a ton of women runners, i can tell you what sends a message to me. When the other runner absolutely does not make eye contact and looks straight ahead, that sends a message. And the message is even a little cold, but i figure that's the woman's way of telegraphing that they DO NOT WANT TO BE BOTHERED. Alternatively, the eye contact and smile is interpreted exactly the opposite way.
Either way, I am the last person on this planet that seeks conversation or anything of the sort - i am just saying how it's interpreted by this guy, anyway.
And in that vein, running with earbuds is just a built in excuse to ignore everything, half of which you may never know ever happened. Not only does it send a message that you are in your own world, you can also pretend that you have no idea that that other person was trying to interact.
Happy runs to you, and i hope that they never become something you have to think twice about embarking upon.