I think this is a case of rumps foresight (and perhaps a bit of Blue Fairy manipulation).
Remember that BLUE FAIRY is the one who told him a curse was needed. She lied about their being no magic beans and I wouldn’t be surprised if she lied about the magic tree being the last of its kind. He casted the curse because she told him that was the only way to get him back. Im sure Rump was looking for other portals (he asked for the ruby slippers for example) but I think he was certain the only way it would happen was if he used the dark curse which again, was said by the Seer to be the way he would eventually find Bae. So I think Rump figured that even if he found another portal, it would not lead to him finding Bae, that only the curse would do that. I think its all bull though. I think Blue Fairy made it that way.
"I could have the giant duck as my steed!" --Daniel Radcliffe
Keeper Of Tamara's Taser , Jafar's Staff, Kitsis’s Glasses , Ariel’s Tail, Dopey's Hat , Peter Pan’s Shadow, Outfit, & Pied Cloak,Red Queen's Castle, White Rabbit's Power To World Hop, Zelena's BroomStick, & ALL MAGIC