He's dick chasing me- Help!

We've been on and off forever. Now we are friends with benefits again... OMG he is determined to screw my brains out and while I want to say yes I want to change. I know I need to walk away if I want to be married someday... But he's sexing me, talking dirty, and fulfilling my sex needs. Sis can't escape because when I turn around there he is chasing me with his dick ready for action!!! I found myself giving in over and over... What do I do? Help please...