Your boyfriend seems to think that he should be able to have females friends separate from your relationship when you on the other hand feels it's disrespectful to have girl friends that you know little or nothing about. It's obvious there is trust issues there on both of your parts you don't trust him to be just friends with these women especially if he cannot share the content of the texts or conversation and he doesn't trust you to see these things because he may think that you will overreact to the content and if that is the case then he really shouldn't be talking to females if he doesn't trust for you to see the conversation also. There is also a lack of respect on his part that this is really bothering you and affecting your relationship. You can tell him until you are blue in the face that his is bothering you but unless he has respect for you and the relationship he will not change the current situation.
Some men can be friend with exes and it not mean anything but friends, he may be the type of person that doesn't like to have enemies and likes to keep things positive but if that were the case he would be able to allow you to see the content just to put your mind at ease. He may also feel that you are trying to invade his privacy and that he feels his friendship with these women should be kept separate from your relationship with him. He wants you to trust him but he hasn't given you any reason to trust him because he is keeping secrets and that is never a good thing in a relationship. Sometimes women feel they can have relationship with other men and exes because they usually do not cross any lines but it would be unfair to expect him to share everything he talks about with these women with you and then you not have to do the same. What you need to do is trust in him that he isn't doing anything that will jeopardize your relationship until you find concrete proof that you have anything to worry about.
Unless you want to be a detective and try to find out things on your own and then take the chance of mess up the relationship for lack of trust all you can do is trust him and hope that he isn't doing or saying things that would break the trust in the relationship. These are issues he is going to have to correct and respect that you have an issue with him talking to exes. I do believe that men and women can be friends with exes and nothing has to be going on but if he is hiding things from you then that makes it an issues.