I think that 90% - 99% of all males would have sexual fantasies about the woman that they are dating even before they've had sex. Didn't you fantasize about having sex with him prior to actually having sex with him?
I think that most males would masturbate while thinking about the woman they were dating. Think about this. If he was going to masturbate, would you prefer that he be thinking about you, or about some other woman????
Although most men would masturbate while thinking about the woman they were dating, not all men would share those thoughts with their woman. I feel that it is a sign of his real trust in you that your bf shared these thoughts with you.
I know that if I had a girlfriend who told me that she had masturbated while thinking about me I would feel very flattered, and also turned on.
Best of luck in your relationship.