There are several reasons for dry mucous membranes. A very common reason is due to washing your genitals with soap. Can we just decide once and for all that soap doesn't belong down there. Possibly a unscented oil, but never soap. There are some rare rheumatic diseases that can cause dry mucous membranes, but in that case it's not limited to genitals but even eyes, mouth, nose etc. You can try applying estrogen locally in vagina to see if that helps. It's prescription free and harmless. If that doesn't help, it could be hormonal. It's very uncommon though if you are still menstruating.