Gay marriage as a political issue should have nothing to do with the bible or religion.
And even less to do with religion when it is two fictional characters getting fictionally married.
Sure, people say, well these comics are promoting something they are against. They also have promoted Human/Android marriages and a MASSIVE amount of violence, some murders (sometimes on a large scale), people steal things, there have been characters hooking up for years so there is out-of-wedlock sex, there have been a few love children, as mentioned by someone else in another post more then a few instances of Rape, kids fighting with their parents, a little paganism (there are witches in the books after-all).
So if someone is religious enough to not want to read a book that has gay marriage in it, they should have already not been reading the book for breaking pretty much every commandment in the past.