Okay, first I want to explain a few things about Poison Ivy so that we can better understand her sexuality within the context of her character development.
Poison Ivy is an ecoterrorist and a straw feminist. A straw feminist is a character who is labelled as a feminist only in a effort to ridicule or prove feminism wrong.
I should also mention that there is such a thing as an ecofeminist, who believes that the oppression women have suffered is similar to the suffering and oppression inflicted on the environment. This form of feminism, like most other forms, dislikes a hierarchy that puts one group above another, whether it’s men over women or humans over the environment, and recognizes more of a symbiotic relationship of all things.
So, Poison Ivy, at her worst, is probably a straw ecofeminist. She hates all men and cares only for plants, but in later years, maybe in an effort to down play her original straw feminist bent, Poison Ivy starts to hate all of humanity. In current incarnations Poison Ivy herself has morphed into a plant like human and she believes that humanity should be wiped out since they harm nature. Despite her hatred of all humanity, there is one person Poison Ivy does seem to care about, Harley Quinn.
It would be easy to simply claim Poison Ivy is asexual since she seems completely uninterested in people. Despite often seducing them to achieve her own ends, Poison Ivy never seems to have genuine attraction or affection for anyone. It would also be easy to claim Poison Ivy is a lesbian since she only seems to care about Harley Quinn, but things are much more complex than that.
Before she was Poison Ivy, she was Pamela Lillian Isley, a botanist working with Dr. Jason Woodrue who seduced her and turned her into the plant-human hybrid she is today. Woodrue is the reason that Poison Ivy hates all men. And though I stated that eventually her story is changed to hating all people, Poison Ivy still has a special hatred for men. Batman, Two-Face, Woodrue, and especially the Joker are targets for her wrath against men. That doesn’t, however, mean she’s not attracted to men. As mentioned earlier Poison Ivy’s hatred of men is there to paint her as this straw feminist character, but she is still obviously interested in men. She liked Woodrue before he betrayed her, and in season two of Batman: The Animated Series in the episode “Home and Garden” Poison Ivy married her very male doctor. Yes, he’s later revealed to be a plant, but she could have made a female spouse if she was more into that.
So, I really think that Poison Ivy isn’t necessarily a lesbian. She has just been given this male-hating persona, which somehow equates to lesbian, sadly. Even in recent years making her hate all people hasn’t changed the fact that she defiantly hates men more.
Later, Poison Ivy is often paired with Harley Quinn when the two form an unlikely friendship. Harley is the only person Poison Ivy seems to like or care about. Though it’s never explicitly stated that they have a sexual relationship it is heavily, heavily implied.
They are often shown scantily clad, or naked together, and even Batgirl has questions about their relationship. They both seem to really care about each other too. It has often been remarked that if it wasn’t for Harley constantly intervening, Poison Ivy would have killed the Joker a long time ago in order to protect Harley. And though much of their relationship could easily be attributed to a male lesbian fantasy in comics, I tend to think that they actually have something.
Poison Ivy seems to be stuck in the friend zone with Harley. I do think that Harley and Poison Ivy have maybe experimented sexually, but Harley if anything is more Joker-sexual than bisexual and her devotion to the Joker won’t allow her to make any commitment to Poison Ivy. Harley is beaten and almost killed by the Joker and then runs to Poison Ivy’s comforting arms, Poison Ivy then wants to kill the Joker to avenge Harley, but Harley stops her and goes back to the Joker–and the cycle continues.
There is no concrete evidence for their relationship, but I think there is a lot of circumstantial evidence that shows Poison Ivy loves Harley Quinn, maybe even despite herself, but Harley is too involved with the Joker to see Poison Ivy as anything but a friend, thus bringing about the central conflict in their relationship.