Men by definition are masculine.
The word masculine brings us images of a male human with square shoulders, a V torso, strong arms and legs and a very square jaw. A man. The Alpha Male. When we write male characters this is the image that is automatically imagined by readers. We then write about the color of hair, eyes, how handsome they look in a loincloth in a romance novel. How virile and strong they are in an action novel. There is no difficulty writing about a Straight Man.