I think he loved her when he had no one else, and there are strong ties to family with Cardassians (even if his other family left him for not killing her- that one of several times).
But at times he clearly only was using her to get to people like Kira, and you can't turn real love off and on like that.
I disagree that he was using Ziyal to get Kira. I think that he and Kira really did have a common bond over Ziyal, one that had begun from the moment Ziyal was found and Kira convinced Dukat (who was close to making an honor killing) not to murder her.
Then Kira took Ziyal off his ship to give her a more secure, stable home, and vowed to protect the girl. This is when Kira and Dukat were "getting along" in their own way.
The bond they shared over Ziyal was real, and Dukat didn't want to use Ziyal to exploit Kira, he wanted to increase the bond between himself and Kira, which had started over Ziyal.
They were happy and got along as strange parents to Ziyal, and though Dukat was indeed a villain character, he was very familial (as are many Cardassians). I can see him deciding that Kira, Ziyal, and himself was the family unit he desired, and attempting to strengthen that.
But just to get Kira in bed? While his methods to attract Kira were uncouth and holdovers from the Occupation, I doubt he was just trying to get her in bed. Besides, he knew Kira wouldn't fall for something like that, and would be too smart to try to trick her. I think he honestly wanted something more, and was too much of a fool to know how to go about getting it.