Well, the Monogatari series is really a complicated thing to swallow all at once. Even I admit that the dialogs, particularly Koyomi's long one against Black Hanekawa during Neko Black Part 4, needs a few rewatches before one can truly grasp what he means. But that's just how Nisioisin rolls.
Based on Koyomi's conversation with Tsukihi and Karen, his feelings for Tsubasa doesn't cover both of the Fire Sisters' definitions. He doesn't want to be together with Tsubasa (unlike Tsukihi's definition of love -- Tsubasa's personality can be disgusting in certain aspects and it will not be helpful for her development) and he hasn't reached a point that he wants Tsubasa's babies (unlike Karen's definition -- although he does want to look at her pantsu and grope her breasts).
What we just know is that it's "something else". From what Bakemonogatari tells us, that "something else" is for the better of the two.
