Am I in the wrong to be upset about my man making "back up plans" with a soon to be married woman??

So my man has a lot of female friends, which has never bothered me due to the fact that I know the majority of them, but there is one in particular that I dont know that he talks to regularly, recently he had me reply to one of those text for him, and the previous text was her saying " if it doesn't work out with my fiance I got dibs on you" and his response was "deal 👌" I immediately got upset, and questioned him about it, I mean this girl is literally scheduled to get married in less than a month, and he and I just had our first child together. He thinks I'm being dramatic and that I have no reason to be upset.... he claims that they've had this deal that if they were both single by a certain age that they would just be with each other, but the thing is hes definitely not single, and neither is she... So am I wrong to call him out and be upset?