The last thing your boyfriend wants to think about is another guy has been with you (whether we're talking about sex or just hooking up). Envisioning some other guy getting down with you can literally drive him crazy.
Have you had a checkered sexual history that's filled with lots of experience? If so, your boyfriend might have trouble dealing with this. But this is something that he must accept.
Don't you dare let someone else throw your past in your face or hold it against you. If it's ancient history (or only a few months ago), let it stay there, and move on.
He must accept you for all that you are. If he's chosen to be with you... he needs to accept you for all that you are. And that means everything. It doesn't mean he has to like your sexual history, of course, but it means he can't hold it against you. It's completely unfair for him to want to date you and then start knit-picking about details from your past.
If he's constantly bringing up your past, tell him flat out that he's going to have to accept everything about you if he wants to date you. Let him know he has two choices:
a) You two break up
b) You two stay together... but he can never mention your sexual past ever again. Never. Not even when you two are fighting.
Learn from your mistakes and move on. The few moments of pleasure that come from sex and hooking up can have a permanent effect on your reputation and self-esteem. If you've chosen to change your "free-loving" ways, you're doing the right thing and that's what matters most. On the other hand, if you've only had a few hookups and your man is making a major issue about it, you might want to rethink whether you really need to date such a puritanical person.