Firstly, it’s completely your choice what you want to do. But, I would suggest that you shouldn’t sleep with your boss just because you want a promotion. If you have really worked hard for getting a promotion then you will get it no matter what. And even if your boss won’t give you a promotion by playing dirty, still it won’t harm your dignity. You might get a promotion a little late than others but sleeping with your boss for promotion is not right. You have to stand up for yourself and strictly say no to her.