Here's why:
* "Ours" is a possessive adjective (like "my," "your," "her," etc.).
* In French, possessive adjectives agree in gender and number with the noun they modify.
* "Ours" refers to something belonging to us (we), and "we" is considered masculine in French.
Therefore, "ours" is masculine even though it refers to a group that might include both men and women.