When things had been going wrong with my first boyfriend for several months, I told him the truth about the last straw: That I had fallen for his friend who'd shown far more desire to be with me and take care of me than he had.
Then I was painted the ***** because when asked why we broke up, he'd say I dumped him for his mate. No "I've been telling her I haven't been happy with her for months", no "I called her boring and that I couldn't stand the idea of being together forever", no "she's been going home crying every time she's come to visit me for the past few weeks because I make her feel like shit".
So, yehknow, it's all very well being honest about why you wanted to end it but be careful that your ex isn't going to conveniently forget their own contributions to the breakup.
As for rejecting people who ask me out, I haven't been single a lot so I can usually very honestly say I have a boyfriend. Often I'm not believed, and guys feel the need to 'test' me by barraging me with a load of questions about my partner.
But yes, it's not any girl's responsibility to explain herself for not wanting to date you, it's probably that they simply don't fancy you. It's hard to get someone romantically interested in you when you're simply not their type, and they really can't give much more of a reason for not going out with you than "... I just don't wanna".
But good news! Girls have more varied tastes than guys do, so while 90% of them might be turning you down, there will be girls to whom you are exactly what they're looking for! http://www.eurekalert.org/pub_releases/2009-06/wfu-ra062609.php