How do you get to know people by writing erotic stories?
As to your feelings, you may feel anything you like, and it sounds to me like he wasn't clear that he would be, and was, still in contact with her as a thing between the two of them, rather than as a joint...friend?...toy?...as you were led to believe. That becomes a violation of trust. That's a problem. If there are continued violations of trust, you eventually won't have a very good relationship.