partner wants to get pregnant together

So me and my fiance want to have kids in about 3 to 5 years and since we both have ourselves vaginas they want us to both get pregnant at the same time so we can have "twins" together but I really really don't want to. Were both going to be first time parents and two newborn babies feels like a lot of work, especially since my fiance works in logging camp and if be at home doing most of the child rearing. not to mention, let's say I get pregnant with twins, then what? Wed be having three babies with absolutely NO parenting experience whatsoever! My uncle lived with me with his twin girls from their age of 1 to 5 and I could barely keep up just helping out occasionally and baby sitting sometimes. not to mention that I have a legitimate phobia of giving birth and being pregnant, but I'm completely willing to suck it up to start a family together.

I just.... I don't wanna have 2 possibly 3 babies right off the bat as first time parents but my fiance refuses to listen to my concerns seriously. I even suggested a compromise where one of us has just one baby and then a year or two later we can both get pregnant to have the other two babies. I also offered to carry a baby or two made from their eggs in case they're worried about continuing their bloodline and doest think they'll want to get pregnant later. hell, I even said I'd consider one of us conceiving once the other was in their third trimester so at least that way the first baby would be not so new when the other came. but my fiance has rejected all of these compromises and refused to even negotiate. what should I do?

TLDR: I'm marrying a female and my fiance wants us both to be pregnant at the same time, but we would be first time parents and I can't handle raising two babies with zero experience. help?