There have been some really good points here. One thing I thought I would add is that this might be something which could change with time. That is, he may feel more secure in your relationship after you are more committed; then people you date may not feel like competition.
I am suggesting this because I understand the difference in his feelings towards your husband and the new people you (might) date. I started seeing my girlfriend recently, and we are both married. I experience no jealousy about her husband, because I regard my relationship with her to be in a completely different level. In her marriage there is that deep-level commitment and understanding which you build with time. In our relationship there is the exitement and enjoyment of finding a new person who is compatible. Now then, if she were to find another partner to start a relationship with, I would propably feel some jealousy because I would feel that he/she would sort of give her similar things she gets from me. Therefore, I think the situation will be different in a couple of years when we have had time to get to know each other better.
Now, this might be the situation for you boyfriend as well, or not. It could help to see, why your boyfriend feels threatened by people you date but not your husband. It may be that he only needs time for you two to establish a relationship before you see other people, or it may be that he'll never be okay with it. You need to find out.