Anne Rice did it waaaaay before Twilight was the wet dream in the author's head. Even Dracula had crazy bitches in it. Sexual taboos are more freely explored in fiction, and the supernatural turns fiction up to 11.
I'm pretty sure if I was a dude I would still think vampire Kiefer Sutherland was extremely fuckable, regardless of general gender preferences. That sexy evil laugh.. gives me chills just thinking about it.
He was a very unique and cool looking young man. When he got older and started doing 24 I couldn't believe that all the attitude, and all the coolness factor was gone.
But to your point, when straight dudes think another dude is cool, or handsome, they don't want to bone them. They want to be like them, or befriend them, or sometimes fight them. We don't always make sense.
All the same vampire stories have always been about same sex people feeling wrongful desires as the suck on each other's necks. It's a genre about being gay and or kinky.