The thing is this: the fellow knows that you are straight. You have made that abundantly clear. You are straight, you are going to remain straight, and no amount of pressure from him is going to change that.
What I am seeing is “jerk” or “obsessed.” It does not matter his gender or sexual orientation. A woman who did this when you said “no” would fall under the same category.
Either way, you are going to have to be less pleasant than you have been. I am guessing that you have tried all of the nice ways.
I would say that it is time to tell your boss, if you are at work, a teacher, if you are at school, and use what authorities are available to get him to cease and desist. Tell him that what he is doing is harassing you and you can, and will, press it legally if he does not desist. If it comes to it… restraining order.