Here's why:
* Possessive adjectives in French (mon, ton, son, notre, votre, leur) are not gendered themselves. They simply indicate possession.
* The gender of the noun they modify determines the ending of the possessive adjective.
Examples:
* Leur livre (masculine singular): Their book
* Leur maison (feminine singular): Their house
* Leurs amis (masculine plural): Their friends
* Leurs voitures (feminine plural): Their cars
Therefore, you cannot say "leur" is masculine or feminine. It depends on the noun it describes.