OP I kind of was on the same boat as your boyfriend. So if you can please listen to my advice it would probably be sound.

So I was 18 when I was in a LDR with a girl in a different continent too. She was older than me, 23 so obviously right off the bat I knew she had more sexual partners than me. For example I had a decent amount of sexual partners for an 18 year old but it wasn't as much as her which is something I had to be understanding as I was willing to go into a relationship with her.

To make a long story short while my ex girlfriend never really told me about any experiences like you have she definitely told me some things I think were better off not being told, for example "I used to be with a barrister and the next morning he would put on a wig", when we were talking about lawyers. So yeah shit like that I probably didn't need to hear.

So my best guess is your boyfriend is probably just as insecure as I was. Which he has the right to be while it isn't bad and your past isn't a problem to you it is something that will probably make a future SO feel uncomfortable with which isn't rocket science. I was much worse than your boyfriend because then I would talk about my ex's and make her feel uncomfortable whenever I got the opportunity. The problem is you are long distance so the only thing your boyfriend really has to go off of you and inspire him about you is the memories you guys have together and the thoughts of you since you can't be there right now. So it is probably hard for him that these good thoughts are being contaminated by not so great thoughts which is allowing him to have a cloudy judgement. Basically talking about your past sexual partners could be one of the worst things to do in a LDR when you can't see the person right away because it does damper their judgement and thoughts.