It was never about sex. It was about dominance and power. He wanted to see Jasmine squirm at his feet and worship him. Jafar probably wanted this from everyone but the princess openly shat on him several times so he wanted to teach the brat a lesson about humiliation. Sure DAT midriff was legendary but Jafar is that type of guy who is all about power. Princess Trainer got this right where he basically lends Jasmine to you (and half of Agrabah to fuck because he wants to see her broken.>>105521456
Marrying Jasmine required him to step out in the open and reveal himself by hypnotizing the Sultan. It was a desperate last resort plan because he always had a backup plan and always played safe. It backfired right away as he may expected it. It seems the staff is only good for one people at once and this makes it a huge risk for a big takeover operation.