Ivy's also, like the Joker, a dominating personality used to controlling everyone in the room. Harley tends to gravitate to those, and before they confirmed anything we saw Ivy take interest in women in some fashion. She ensnared Selina in Hush and kissed Supergirl in Superman/Batman...
Both written by Loeb, come to think of it. Also, wasn't Kara a minor in that story?
But yeah, it had been theorized for years and-- at most-- Ivy had some repressed feelings for Batman that didn't get touched on much. I still think it's strange that Ivy seemingly lets Harley do what she pleases and control the direction of their relationship given Ivy's entire publishing history saying she wouldn't, but that's what happens when you get turned from your own character into Harley's +1.
Regardless of the relationship, I have difficult thinking there's complete consent with Ivy involved without seeing that relationship hurdle cleared on panel. I don't see someone like her surrendering that much control to anyone, though I suppose the argument is that Harley's so battered perhaps Harls triggered a protective quality in Ivy that made something of an exception for her?
No matter the case, there was more precedent than some give them credit for. It wasn't just that they were both friends, female and that was enough. It bugs me when any two characters with significant relationships are shipped for no other reason than, well, because but with those two my only issue is the interesting thing about their dynamic got skipped so we can have a healthy bisexual relationship. There was a cool story there and I feel like we got robbed of it. Much like Harley's redemption arc, but that's another subject.
If you're a newer fan, especially post New 52, Ivy's always kind of just been a Harley Quinn supporting character and nebulously anti-hero so anything resembling a more classic approach would be looked at as character assassination. I think the ship has sailed for wanting to see them work out their differences and make it work. They just magically clicked I guess.