The email addresses for graduate students are often posted on department websites--go ahead and email students. But, it's best to have these conversations over the phone because students might be hesitant to be honest if it requires putting their thoughts or experiences in writing, so you might want to email to request a chat via phone. This isn't a strange thing to do at all. Prospective students do it all the time. What I would email the chair to ask, though, is whether there are any demographic differences in attrition and completion rates, if any women have dropped out (and if they have, does the department have contact information for them so that you can talk to them about why? Sometimes the reasons are totally innocuous, but sometimes they aren't). What are the demographics of the faculty? Who do they invite to campus for talks? It's worth talking to students from other social groups in the program too (do they care about equity issues? A program can be mostly male and still be quite friendly to women). If people are hesitant to talk about gender issues, race issues, etc., that's a red flag.