I agree with SNM. Many women do get turned on by M/M romance, with or without spanking.
I used to be really into fanfiction, particularly Harry Potter fanfic. Of course I've looked up just about every HP fanfic involving spanking. About 80% of them are M/M. The rest are pretty much M/F, with the odd F/F or F/M story here and there. Nearly all of it is written by women.
It's not just about spanking either. I've spoken to many women who love to admire and read about gay men. One of the online communities I follow is about figure skating, and many of the women there love to fantasize about male skaters whom they know are gay. Some even write M/M fanfiction about them!
I don't know how common it is, but the interest is definitely there.
