Don't say it was a dare. That just makes you look immature, whether it was actually a dare or not. But also don't say that you really like them and that you're really upset about this because that would just make them feel bad. Even though maybe you are upset about it and you want to make them feel bad, don't do it. It doesn't do you any good anyway; it's not going to make them change their mind.
Instead, say something like "oh okay, I understand. It's all good. Thanks for hearing me out anyway" or "it's cool, no big deal." Try not to show that you are hurt at the time and simply smile. When you get home, then go ahead and feel upset, you deserve some time to let it all out; just don't do it in front of them or else it'll make them feel bad, doing nothing but trouble. And when you see them around after this, just act natural. Don't bring up the whole asking them out thing again, but also don't avoid them. Just try to be the way you were before you asked them out and act happy. They can't help feeling the way they do, so don't do anything brash. If you make yourself seem fine and they think that you are fine, it'll be easier to get over it and actually feel fine.