As usual, I've got to differentiate my fantasies from my writing. When I'm fantasizing, it's usually a one-on-one experience. I don't LIKE sharing, personally. My fantasies usually involve a single woman who ends up (or sometimes starts out, in a mind control fantasy) worshiping me.
My writing is a whole 'nother story. When I'm writing, the attacker need not be me, and the victim need not be anybody I know or have any desire for (well, if I don't find her at all desirable, I won't be writing about her; but you get the idea, I hope). Therefore, though gangbangs aren't exactly part of the business day in my stories, I have no problem with using them if they seem appropriate at the moment.