She's lived in a place full of women all of her life. Why wouldn't she have found love with another woman? And it's Greg Rucka. Rucka in on par with Perez in how much he respect history and continuity of Wonder Woman. Revealing that Wonder Woman is bisexual is fine and it's pretty much in line with a lot of Wonder Woman's crazy history of magical twins and fighting time-traveling Nazis.
Diana has never been shown to be attracted to women. In fact, when Steve arrives, it's surprising that no other woman is attracted to him at all, besides Diana. I always thought that the Amazons had evolved past that physical, sexual need, or else Paradise Island would be rife with many of the same ailments as Man's World.
There's a fair amount of hypocrisy in how Themyscira is approached, as I'm certain if the situation was reversed, and a woman showed up on an island of enlightened men, readers would guffaw if not one of them was attracted to her. It would make sense that a man is attracted to a woman, but the converse? Many don't seem to believe that women are attracted to men, and deeply at that.