My fiance likes to tango. I don't really care. But from what I understand it involves a lot of hip against hip, looking deeply into some guy's eyes, and dancing romantically with them.
No I wouldn't care, ask to go to watch. Ask to meet her dancing partners. If she's sneeking out and being dodgy about who she's meeting and dancing with then yes there could be a problem. But if she's open about it and has a group of people she goes with I wouldn't worry at all about it. I would go occasionally just to show your interest in her interests.