It depends. I am a 31 year old female and I have never wanted kids. If I were a man I think I probably would want them. Statistically men are more certain they want children than women. The reason for this is that girls worry about pregnancy and the responsibility of mother hood from an early age so they put a lot more thought into it. As a female you lose alot more than a male when becoming a parent. Your career will be affected, your body and mental state will be affected (of course dads have stress too but I mean hormonally), you will be judged about taking maternity leave, you will be judged about going back to work too early, you will be judged on breast feeding etc etc. This is why most women who want children do not want them in their early twenties. Having an urge to have children is not biological no matter what people say, there is no hormone that makes you want children, women have an increase in sex drive caused by testosterone which is the hormone responsible for ovulation (hence why we take female hormones to prevent pregnancy). That is the bodies way of saying you're ready to get pregnant.
As a young male you are probably loving the idea of impregnating someone because it is in a way confirmation of your masculinity. However after pregnancy, comes a child, sleepless nights, crying, arguing, saving for clothes, education, toys, food etc. You arent just knocking someone up and having a baby. You are creating another human being who will be solely dependant on you for at least 20 years. Possibly even younger. I have watched my friends struggle, in tears at the arguments with the kids and their partners, I have watched friends and family relationships break up, i have seen kids heartbroken because dad can't handle it and has walked out.
Of course there is joy and love and great times but it's not a walk in the park. It is hard and expensive.
Your fantasies are normal. And it's good you know you arent ready yet. Try and only bring a child into a stable meaningful relationship and enjoy your youth, the time will come eventually. But take my advice and never pressure a woman into it and don't remain in a relationship with someone who doesn't want kids. More than likely they will never change their mind. Trust me, I know from experience.