The relationship between Harley and Ivy is that something to prop up Harley. Harley is a character that is defined by her abusive relationship with the Joker. Once the fandom wants to Harley get away from that, there is no reason to put her in another relationship with another controlling partner. However Ivy is not a saint. If you go with her new 52 origins, she was a child who suffered abuse from her controlling father. Then with the post crisis backstory, she was seduced and poisoned by a man. These tragedies propel her to become a manipulative and violent individual. Ivy went from being a victim to being a victimizer. Ivy sees people for how useful they can be to her or how fitting they are as victims for her poisoning (and her other painful killing methods). Look at how she poisoned the BoP to make them work for her and killed the male drug dealer that traveled along with BoP members. The relationship between Harley and Ivy is depicted to be what they are because again, its a popular ship and DC has to change Ivy for that relationship to work in a healthy way. But the nature of Batman world is that it is full of broken people. People that can never let go of their traumatic past just like how Bruce can never let go of the night where he was victimized by a criminal. Is Ivy incapable of caring for a human? No. Ivy can be very protective to the thing or the people she cares about. But there is always that brokeness in the character that should make her not be able to have totally healthy relationships with others. In Gotham Knights, the orphan kids that she took in during NML ended up poisoned by her. Ivy unconciously poisoned the kids that she took care of.
Batman is drawn to broken people not because they are as broken as him but because he wants to heal them, he wants them to not be as broken as him. Its why i was frustrated when i saw Ram V write Batman say to Catwoman to steal something important because thats the last thing Batman would want to say to Catwoman. Batman knows Catwoman is a thief that finds enjoyment in stealing from others. The difference between Batman and his villains is how he copes with his own trauma versus how his villains cope with their traumas / mental illnesses. The traumas, mental illnesses that the villains suffer from exacerbate their evil while the trauma that Batman suffers from makes him want to prevent others from suffering like he does. When villains suffer from depression, they cope with it by committing more crimes because the villains find the most enjoyment out of committing crimes than doing other stuffs.