I always wondered how guys handle it when women wear sexy costumes etc. I assume that you two are dancing Latin or Rhythm dancing.
I know that many of the professional male dancers have to deal with issues like this when they dance with their students. And from talking to a few of my pro friends, I know that it has not always been easy for them not to respond sexually when doing a provocative dance with a partner in a provocative costume.
I am assuming that you and your partner are both amateurs. If you were a Pro and the woman in question is a student, then I would encourage you not to date her, not to take it any further than dancing and to try everything you can to keep it professional. I wouldn't even let her in on the fact you were responding this way, I would just encourage you to find ways to deal with it.
If you are both single and amateurs, then you have other options. As a woman, I can tell you what I would want you to do if you were my amateur partner. (Mind you, this is only from my perspective, because all women are different and I tend to like a direct approach.) I would want you to talk to me about your feelings at a time when you could be doing it from your head rather than during the actual moment of emotional intensity. Something along the lines of, "You know I find you very attractive and sometimes it distracts me from our dancing. If you were interested in dating, I would love to pursue these feelings. But, if you are not, I wondered if we could talk about this a bit. Perhaps you could do a few things to help me, like perhaps saying something humorous, if you know that I need a little cooling down. (Or don't be surprised if bring up some humor in the middle of an intense moment in our practice) Or just remind me after a very intense dance 'This is only dancing, we can't take it off the dance floor'. And a special request, could you perhaps not bend down in front of me to do your shoes. The view is a bit too lovely for me."
I know that this may seem way too direct. But if you want to keep a dance partnership, sometimes you just have to talk about the feelings intellctually and this sometimes helps you diffuse them a bit. And also, there are things women can do to make it easier on a man. Sometimes a woman is just unaware of the things she is doing that are provocative off the dance floor. I know that I always bend over to buckle my shoes and am simply not thinking of the view that might present to someone standing close by. If I knew it was a problem for my partner, I would find ways to be a bit more conservative about it.
When I was dancing Latin with my last partner, sometimes things got a little hot because of the choreography, which was provocative. My partner was gay, and I am not. And I am married, so I have no intention of taking those feelings off the dance floor. So I talked to him about the fact that I was attracted to him at times and didn't want to act on it outside of dancing. And so, when we had those intense feelings and the dancing was really good, we enjoyed it. If it got too hot, one of us joked about it. After all, there was humor in the fact that we were sometimes attracted to each other and he was gay and I was married. There wasn't any place for the feelings to go except into the drama of the dance. So we became partners in trying to use those feelings to create better dancing. I remember one practice where I stopped us and said, "Whew. Let me recover for a moment. That was really HOT!" And I walked around the room for a few minutes and cooled off. Then we concentrated on the technicalities of the dancing, which is usually enough to kill the passion by itself, because it takes so much concentration.
Dancing and loving it,
Karen