Help required. Sex with a boss.?

Well this guy works above me. One day over Facebook chat, he asked me for sex. It began as a joke. However, I couldn't refuse as I was kinda tempted too. Somehow, we have been postponing the session for a month and half due to circumstances. Now, I really need the promotion but that can only happen in November. Will having sex with him affect my career? How can I get out of the situation?