From Idolatry to Conversion: Judaism and Non-Jews: Idolatry and Non-Jewish Practices: Cross-Dressing:
Why is cross-dressing forbidden?
Maimonides, in his law code Mishneh Torah, lists the cross-dressing prohibition
of Deut. 22:5 (which forbids women wearing clothes that are ordinarily worn
only by men and vice versa) under the category of laws relating to idolatry and
the customs of the nations. This suggests that cross-dressing may have been
associated with some type of idol worship, or may have been practiced by some
group of idolators, and was forbidden by the Torah as a guard against these
practices. Note that what is forbidden by this law is only the wearing of
garments that are ordinarily worn ONLY by members of the opposite sex. Thus
the prohibition depends on how people dress in a given country at a given time.
If the wearing of skirts by men became widespread (like the wearing of kilts by
Scotsmen), it might no longer be forbidden.
Ask a follow-up question