There are many ways to play the game and you don't need to have Sim babies if you don't want to however, if you plan on playing a legacy game in The Sims 4 with multiple generations of the same Sim family, you'll probably want to have your Sims trying for babies pretty regularly. Your Sims stories need to continue on, and the only way to do that is your Sims getting pregnant and carrying on their legacy. The Sims 4 might not be the perfect life simulator, but it gets pretty darn close - especially with how it handles pregnancy.
