From my personal point of view I can totally understand why you are feeling the way you do. When my husband & I decided to open our marriage up so to speak & allow someone else to be a part of it, it initially started with a relationship w/ him & a close girlfriend of ours and I was not involved whatsoever.
However we found that this type of relationship did not work well for us as I felt very left out like a 3rd wheel even though my husband was being very attentive to my needs & making sure what I needed & wanted came first. After a lot of talking he also agreed that he wouldn't feel comfortable with me dating someone separately from him and so we decided that we were better suited for a closed triad relationship. Once we transitioned from it just being him & her to it being him, me & her things were wonderful & amazing. Though it sadly ended it was a learning experience for us both.
It's natural to feel insecure & a little jealous. And it's good that you are all keeping the communication open & honest, that's extremely important. And there's nothing wrong with stating that you are the primary & that's how it must be, everyone has different needs/wants & what they are/aren't comfortable with. For my husband & I we choose to see ourselves & whomever we're in a relationship with as all equals, however our marriage & us will always come first & what's best for us & our marriage.
And though you've discussed there be no emotions involved at this stage it's sadly not something you, he or she can really control. When there's frequent intimacy eventually feelings will develop. But don't take that as him loving you any less. The best way I can describe is that your husband is a polyamory person, he has the ability & capacity to love several people at the same time but as someone else said there are different levels of that love for different people....he may love her but not in the way he loves you. It doesn't take away from the love he has for you in any way.
I'm not sure if this would be an option for you but maybe try to be involved in the relationship & form a triad. I'm not saying have sex but just dating & getting to know one another on a more intimate level then just friends. You never know what might happen, it may work or it may not but you don't know unless you try. And being included in that you'll feel a lot less insecure & jealous. Just a thought.