Dude, you are *so* missing the point. You asked someone out, and he said no because he's already seeing someone. That's not really rejection. And even if it were, you're no worse off than you were. Nothing ventured, nothing gained. No one gets a hit every time at bat. *insert other platitudes here*

Seriously, you should be proud of yourself for taking a chance. It will increase your chances of someone saying "yes."