I understand where you are coming from and can empathise with your feelings.
Let's look at some possibilities.
It could be that he is more sexually attracted to her than he is to you. Let's take that as a worst-case scenario.
If he is, this is clearly a problem for you - and that's understandable. Some people don't seem to mind about the sexual element, if other parts are fulfilled. Me personally? I want a primary who is as interested in sex with me as I am in having sex with them. I also want a primary who is equally, if not more, interested in having sex with me as they are with other people.
So.... if that's the case for you, that's a relationship expectation. It's ok to know what you need and to communicate it.
However.... on the plus side..... your boyfriend *could* be saying "we are intimate in so many other ways; sex isn't important".... but he isn't. So, either he's bending the truth, because he doesn't want to hurt you or doesn't want to lose you... or.... he is actually telling the truth.
Onto him having sex with her multiple times, each time. There could be many reasons for this. Yes, it is newer, so that's one thing. That, like everything in life, will probably wear off with time and comfort.
Let's say for a second that he actually does prefer sex with you. Possible reasons for his multiple-sex-sessions with her could be anything. It could be that he feels pressured to perform. It could be that he doesn't see her that often (?). It could be that he is more fulfilled with you sexually and doesn't need to do it multiple times.
On that strain of thought... I'll give you an anecdote. My girlfriend often has periods of sleeping with her husband about once a week. Sometimes twice. When they have sex, it's mainly vanilla, quick, comfortable. I wouldn't want that kind of sex, but she insists that she likes it. Her and I, on the other hand, have sex less often, probably once every two or three weeks, it's kinky, it lasts longer, it's very passionate, fun and intense. I used to be bothered that she had sex with her husband more than with me.... but then we realised something. We realised that actually, the kind of sex we have is so deeply fulfilling, that we don't feel the need to do it every week. Also, because a lot more thought goes into it, it can take more energy and effort. We've agreed that we'd rather have fantastic sex once every 2-3 weeks than mediocre sex twice a week.
When my girlfriend has new boyfriends, she tends to be very sexually focused on them. I have the same fears that you do. I've talked to her and she insists that she does not 'prefer' her sexual exploration with her boyfriends - but still needs it, to fulfill a side of herself. She once said that even though she is never going to give up sex with other guys, she feels that our particular sexual relationship is irreplaceable.
I would consider hearing less about each other's sex lives, though. Certain acts are going to drive us crazy, because we aren't inside our partner's heads - so, we will project our own feelings onto their actions. For example... my girlfriend never enjoyed receiving oral sex, until she met me. I found this so special and so amazing, that I had given her that new experience. Then, she found a new guy to date. She told me that she had given him instructions, using the things that I do to her, to teach him how to perform oral sex on her - and it worked. I hated hearing that. I was flattered; yet, at the same time, felt robbed. Which is stupid. So - better if I had never heard about it.
The same thing for you - you can only project your own thoughts onto his actions.... so.... wouldn't it be better not to hear about his actions?
Overall, I'd say talk to him again. Sit him down and really tell him that you're having an issue, but you just can't feel comfortable with them having a break. The fact that they are both willing to offer it (which is more than my girlfriend would do) hopefully means that you know how committed to you he is. With that beautiful act in mind, you might be able to turn this around. He's offered to stop - but if you encourage him to go for it, you are gaining some control back, in your subconscious mind. A small break might help - whatever you think is appropriate - whilst you work through your feelings and get some more reassurance from him.
Above all - I know that working through your emotions is very hard.... but I really think you will get there. It sounds like your boyfriend is patient and considerate; so that is a wonderful thing.
Me: (30f) open poly
GF: (40f) My long-term, long-distance partner
Hubby (37m): GF's husband
Garcon (26m): GF's submissive/third partner
“Peace comes from within. Do not seek it without." ~ Buddha