There are groups within Christianity who believe that it is wrong for women to wear pants.  Their belief is that pants are men's clothing and therefore a woman should not wear them.

Under the covenant God made with the Israelites (the Old Covenant), He instructed them not to dress in a manner that would confuse others as to one's sexuality.  Men were to dress as men, and women, as women.  As the Bible says: "A woman must not wear men's clothing, nor a man wear women's clothing, for the LORD your God detests anyone who does this."¹

God's purpose in this was to maintain the distinction of the sexes.  This, however, does not prohibit women from wearing women's slacks, or even pants that would normally be worn by a man.  What this verse is saying is that we should not dress in such a way as to fool another into believing that we are of the opposite sex.  A lady should dress as a lady would in her society, just as a man should dress as a man would in his society.

Bible References:

¹ (Deuteronomy 22:5)