Just tell whoever it is "I'm just looking for a dance partner." Then it shouldn't even matter whether they find you attractive- you just told the other person that your only goal in asking them is to dance. It's their problem if they don't get the message. Also like anon says, the better dancer you are, the more options/opportunities you will have in finding a partner so don't stress about it and work as hard as you can on your own dancing first- make that your priority.