OP, in theory there’s nothing wrong with having babies young. My parents were early twenties when they had me and my siblings and certainly now that we’re older I am glad that they were younger as I do fear that with people having children older and older now we are heading towards a grandparentless generation (talking about people having children into their 40’s and beyond...)
But there are numerous things to consider here.
You say that you are in a well paid job and that your next promotion is just three years away. That’s now while you’re young and childless. Fast forward a year when you have a baby, have taken a career break and then another year when you’ve had another baby or are expecting one and that promotion that was just three years away is likely to go to someone else while you’re at home bringing up the kids.
Then fast forward another fifteen years, the kids have left or are leaving home and heading to university, and meanwhile your career has nose-dived due to the fact you spent all your twenties at home bringing up the kids while others were excelling in your career. And you’ll have another 25 years in which you need to work but you’ll be the older generation by then and the 21 year olds of tomorrow, the kids the age your kids will be at will be the ones excelling in your career while you’re doing a general job because you gave it all up to have children so young.
Obviously there are people out there who juggle a career with parenthood, but those are the people who plan to do so early on. It sounds here as if your plan is to give up work to have children and go back at a later date. The thing is, that later date won’t put you back into the career you’ve left, you’ll be as many years behind your peers as you’ve taken out to be a parent and you’ll be at a huge disadvantage but it’ll be worse for you because you’re so young and will have so much more time left to be a part of the work force than your peers who have waited until their late twenties to have children - while they have built their careers in the meantime.
Don’t get me wrong, there’s nothing wrong with staying home with the kids. I did it because I could afford to, and I had mine in my late twenties. However getting back into work is almost impossible now that the DC are older, and added into that mix is the fact that I became a single parent which wasn’t a part of my master plan.
Having babies is lovely. But it’s such a short time in a child’s life. The being pregnant and having a baby is just a year or two of the process. And then it’s toddlers, preschool, having no money because you have to buy clothes for the kids, pay for school trips, and then they grow into stroppy teenagers, and then they leave home and you’re the one left behind knowing all you gave up for them and the fact that they don’t care because they’re off doing their own thing.
Being broody is just a feeling. Getting that positive test is just a snapshot of the process. There is so, so much more to it all than that, but it’s often too late by the time you realise that.
Your partner is being sensible wanting to wait. Realistically leaving someone because they won’t commit to children while you’re only 21 is not realistic. Even if you find someone who wants babies sooner rather than later the reality is that it’s better to wait until you’re in an established relationship before even trying. So that’s maybe another two/three years to then start trying and then maybe another year to fall pregnant, and another nine months before you have your baby assuming all goes to plan with the pregnancy.
It’s far more realistic to tell family that you’re only 21 and there’s plenty of time to be having babies. And then refuse to discuss the subject further.