When dancing with people I don't know, I usually ask whether they have a role preference instead of just going by their appearance. "Do you have a preferred dancing gender?" Ways this can go:

• "I like to lead" or "I like to follow"→ "ok!"
• "I don't know how to lead"→ "want to try?"
• "Can we switch off as we go?"→ "sure!"

Note that these responses use "lead/follow" terminology: that's what people usually use when I ask this. Which makes sense, both because we're not talking about actual gender and because leading and following really is the subject at issue. Briefly:

Some dancers think of contra dance as being directed by the caller, with everyone following a preset choreography. Neither dance role leads the other, everyone just dances. Others think of lead/follow communication and variation as central to what makes the dance fun and interesting. These two groups are much less aware of each other than you might think: in online discussions people constantly enter with comments like "contra is obviously lead/follow!" or "how could anyone possibly think of this as lead/follow?"

Among dancers who think of the gents role as leading and the lady's role as following, the way our language suggests men dance the former role and women the latter continues a system of men telling women what to do. One way to help fix this is to rename the roles to something like port and starboard, but people feel very strongly about these names. [1] Asking people which role they would like to dance is a more personal, flexible, and less controversial way of addressing this problem.

(I think we need both, but this one can be applied by individuals.)

[1] I count 500+ facebook comments over the past week. (On posts by Rowan, Julia, Will, Becky, Rachel, Zöe, Alex, and me.)