Dealing with your partner's past relationships is often a tricky situation. It is normal to feel jealous, but you have to realize that almost every partner you have will have had past relationships. Part of growing is experiencing different levels of commitment and intimacy with different people. You and your boyfriend have different experiences from your respective pasts but you need to focus on the present. The two of you are in a relationship and should concentrate on what makes it work. Past girlfriends, while known, should not take center stage in your relationship.
You and your boyfriend are sexually active which involves many levels of commitment to each other. It is important to be completely honest with one another regarding sexual history, but the details of sexual activity are best kept private. Since you are sexually active it is important that you always practice safer sex at all times.
You may feel jealous and even a bit sad but you have to accurately judge where these feelings are coming from. Past relationships are simply the past and should not shadow your current relationship. Do you feel that your boyfriend is being completely honest with you? Is it possible that these feelings encompass something bigger? If you are in a comfortable and strong relationship a past relationship that is not of the same commitment level as your own should not trouble you.
It may be best for you and your boyfriend to discuss this situation and any past relationships of yours that he should know so that the two of you can put the past completely behind you.