Dear Sarah,
I do not have all the answers but I do know this…if a guy wants you and wants to be in a serious monogamous relationship with you, he will make that very clear to you.  Sex for a woman and a man can often mean very different things. It sounds as if you want things to work with your ex, so I would suggest you ask important questions and really listen to his answers. Be open to hearing what he is actually saying and not what you hope or want him to say. Ask him what he would like to see happen with your relationship. You can follow this up with asking if he sees a future for your relationship. Carefully listen to his words and then watch his actions. If his intentions are to get back together, his words and actions will match that intention. If his true intentions are not to get back together, but just to have fun and enjoy having sex with you then his actions will show that. Please know that you deserve to have a full and complete relationship with someone who truly loves and respects you.