Its very very normal. I think most people feel like this...we cant help asking...we're curious, and yet when we find out we feel hurt.
When you're in love, you sense that your partner is the person they are because of past relationships and friendships...a part of those people will always be in them so you feel like they will never forget that ex, that girl he almost got together with, or worse, that female friend he always fancied and is still friends with!
Its a hard thing to do, but try and appreciate what his ex's did for him. Perhaps his last ex was neurotic, so he now appreciates your fun easy going spirit? or perhaps she cheated on him so he realised how it feels and wouldnt ever do it to any girl now? Perhaps she taught him how to kiss, and if you'd kissed him pre-his ex you'd have been distinctly unimpressed? Ex's can be a good thing, so long as he has moved on.
Think about how you feel about your ex's. Are *you* still in love with them? Still clinging on? Then why should he be? Unless he makes comments all the time that lead you to believe he still likes them that way, dont worry. It is normal to feel jealous, and he may find it cute in moderation. In time to come you may part on good terms, and be his ex, would you want him to wipe all memory of you foreever from his mind, or learn from the mistakes, and remember the good times without longing to be back together?
Lastly beware of perfect ex worries: they werent perfect, and if he makes out that they were, he's being insensitive. They will have argued too, had their differences, and the things that they didnt like about each other. We cant start our lovers again with a clean slate, much as we might sometimes want to, all we can do is not be trapped by jealousy, or get rid of the guy if he's still obsessed.