When I am not interested (which is like 99.9% of the time), my answer is a very polite "no, thank you." And that's it. Pretty invariably I get asked "Why not? (I didn't see a ring)..Are you married, or do you have a boyfriend?" My answer is, without exception "I wasn't aware I had to justify my personal decisions to you." Yes, I get called rude, but I don't really care. It's ridiculous that people are expected to justify why they aren't interested in dating someone.
I usually just say "No, thank you" as well. When I get asked why not, I either say, "I don't understand the question. I need to give you a reason?" with a puzzled look on my face, or I just say, "I don't want to go out with you."
I remember when I was about 21 or 22 or so, a guy asked me out, I said no, and he asked me, "You have a boyfriend?" I said, "No." He persisted. "A girlfriend?" he asked. I said, "No." Then he said, "Why won't you go out with me?" I just looked at him, and answered, "Because I don't want to. I think that's a good enough reason." I hated that he was grilling me about it, so I made up my mind then and there to never make up a lame excuse if I didn't want to go out with someone.
About 20 years later, a guy asked me out when I was married to my ex. We were monogamous (I'd never heard of poly nor had I ever even thought about non-monogamy back then), and he also asked me for a reason and I told him, "I'm not interested in going out with you." When I came home, I mentioned the interaction to my husband. He got a bit upset and said, "Why didn't you tell him you're married!!??" And I said, "Because if I told him that, he would think that if I
wasn't married, I
would be interested. And I want him to know I'm not interested whether I am married or not." I could see on my husband's face, his brain just sort of going haywire for a few seconds on that one.
I guess it's part of our societal training that, since women are supposed to belong to a man, like property, if they turn a guy down, that must mean they are someone else's property already. But if they aren't anyone's property, then they should jump at the chance to belong to someone - hence most men's utter confusion when a woman turns him down if she's technically available.