Is it permissible for men to wear Jewelry? Such as a necklace with a Mezuzah or Magen David? If not, why?

It is permitted for men to wear men’s jewelry in a place where the custom is that men wear jewelry, but it is prohibited for men to wear jewelry in a place where the custom is that only women wear jewelry because of the Torah prohibition "…neither shall a man put on a woman's garment" (Deuteronomy 22, 5). [שבת סב. רמב"ם הלכות עבודה זרה פרק יב הלכה י, שו"ע יו"ד סימן קפב סעיף ה].