So I'm wondering, what's the purpose of a relationship?

I've done okay with girls this past year, and feel pretty comfortable getting them and all that. Of course, I could be getting hotter ones, but that's beside the point. I'm seeing a number of girls now (anywhere from 3 to 6, depending on how often I want to meet them), so I generally feel pretty good about it all. I don't feel like it's any sort of accomplishment aside from personal progress, so sorry if it sounds like bragging. Oh, and they know I'm seeing other girls, since I'm pretty straight with them.

With that in mind though, there's one girl I'm seeing who I'm into quite a bit. With the other girls I'm seeing, they might have boyfriends, or they're married, or if they have another guy as a FWB I don't mind, but with this one girl, I get incredibly jealous thinking about her with someone else. I'm just a jealous guy in general, I suppose. This girl isn't the hottest of the girls I'm seeing right now either, so I don't think that's it. I really enjoy being with her, and am really drawn to the fact that she has a heart for volunteer work and such.

Problem is, I want to have sex with lots of other girls. I want new girls. I have an above-average sex drive, I think. Is there a way out of this? To try to answer the question at the beginning of my post, maybe a relationship is only there to state that you're exclusive to a person, so don't have to worry about them being with someone else?

I can give more info about my relationship background etc. if you like, but I didn't want to make this post too long. As a side note though, I live in Japan, so things kinda operate a little differently here. Tofutofu, any advice?