isn't "I don't think of you as a romantic partner" clear, even with a "you know ... right?" on either side?
No, it's not. The "you know ... right?" makes it sound like you're not sure.
The thing is, words are sometimes used to discuss objective facts, and sometimes they're used for emotional reinforcement, where the actual meaning of the words is less important.
When you stick the "you know ... right" in there, it makes the statement ambiguous in this respect, because it starts to sound like the statement really means "I need emotional support and I'd like you to agree with me" rather than "I don't want to sleep with you". If you mean the latter, say the latter, and don't add unnecessary verbiage.
Are you guys serious? If a girl tells you that she is not interested but tries to be polite and not hurt your feelings, you interpret it as she's not interested *now* but maybe some time in the future?
Look at it from the guy's point of view for a moment. He doesn't know whether the woman can't ever be interested in having any kind of personal relationship with him, or whether she's actually interested, but just not right now. She's already agreed to dance with him, so he's likely to make the optimistic assumption that the latter is the case.
Given the assumption that she's interested, just not right now, what's she expected to do? That's right, she's expected to make some excuse for now, but to save his feelings to keep the door open for later. And that's exactly what turning him down but saving his feelings is likely to sound like, given his expectations.
And then she's "emotionally manipulative", because she keeps saying "no"?
When she adds the "saving his feelings" part, it's not saying "no", it's saying "maybe". It's better just to be honest and straightforward if you want a healthy dance partnership.