Honestly, when I was younger I thought someone so young having kids was insane. I wanted to travel the world, work overseas and have kids in my late 20s/early 30s. People have different goals in life and I used to struggle to understand why someone would settle down so early when it was the furtherest thing from my mind. I fell pregnant with DD when I was 22 and now of course I can see that kids don't stop you from living your life, they just make it different. Your friend is probably just having difficulty separating her goals from yours. I think it's great you have an honest enough relationship that she feels she can share her concerns with you. Don't worry though - when it happens and the baby is here she will forget it was ever a 'bad idea'.
As for the marriage thing - Pfft. We have been together six years, had DD, bought a house and have another child on the way with no intentions of marrying any time soon. That certainly shouldn't be a stumbling block.