Let me put it this way. Lesbians are more useful in an action-adventure novel than gay men. Why? Because Lesbians are more action adventure oriented, they pursue "male" professions such as that Captain Marion Alston in Island in the Sea of Time, being a caption of a ship takes her to the heart of the action, puts her in contact with the natives, and puts her life in peril on many occasions. Now imagine there's a gay man on Nantucket when the transition occurs, what sort of profession you think he would follow.
Let me make up a character to illustrate the point, lets say on that fateful night there was a one Melvin Smith - a gay man, he is a chef at a local bakery, he's also a part time floral designer, he does party planning, and interior decorating. So what does Melvin Smith do when all the lights go out on the night of the storm? He screams, he tries calling the Police to complain about the local black out, he worries about the cut flowers in his floral refrigerator, he panics. Without electricity, his refrigerators will grow warm and his flowers will wilt, oh what's poor Melvin to do? He bites his nails and calls his boyfriend.
Now if you were the author of this book, what sort of adventures can you run Melvin through in the year 1250 BC?
You see the point being, Gay men are effeminent, while Lesbian woman are masculine in their outlook or butch. So you see from Stirling's point of view in writing an action adventure novel a character like Marian is more likely to get into the thick of things than a character like Melvin, who's likely to hide under the table.