Once you start declaring something masculine or feminine, you get into dangerous territory. Because of course there are men who love pink and women who have never met a ruffle they could stand.
There are, however, certain qualities and materials associated with a more masculine aesthetic. Things like flannel, leather and wool tend to evoke a manly vibe. As do colors like gray, black, brown and blue. Straight lines and hard edges read as masculine, as does an absence of bright colors.