Your mother is half right; the Bible does tell women not to wear men’s clothing. See Deuteronomy 22:5: “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.” But her understanding of that text is misguided. Scripture does not define men’s clothing, and obviously the notion of appropriate clothing for men and women is different across time and across cultures. Neither men nor women wear the same thing in twenty-first century America that they did when the Israelites prepared to enter Canaan, when this passage was originally written. The passage is aimed at transvestites, also called “cross-dressers”—men or women who get erotic pleasure from wearing clothing belonging to the opposite sex. Your ex-boyfriend wants to be a transgendered person, not simply a cross-dresser. He probably thinks that emotionally he is what he obviously is not physically—a woman.
Your mother seems not to believe that slacks or pants are now an acceptable part of a decent woman’s wardrobe. She thinks, mistakenly, that pants are still exclusively men’s clothing, as they were one hundred years ago. Be assured, she will not even begin to change her mind unless you wear modest pants or jeans. Many Christian women need to heed the apostle Paul’s words about their dress styles whether they choose dresses or pants: “I also want women to dress modestly, with decency and propriety” (1 Tim. 2:9).