I'd like to further say that reproduction is a choice. During one’s lifetime, people can choose to have children or not AND both choices are valid. Reproduction is not the ultimate responsibility or goal of all humanity. Just because you can do something doesn't mean you should or that you have to. Having children isn't a mandatory experience. A few people here and there choosing not to procreate will not kill off the human race.
Hypothetically, if you believe having children is what everyone should do because they have the parts and capability to do so, I could counter with an observation that most people are born with hands, arms, legs, and feet, so how do you know humanity wasn't made with the responsibility and expectation to play jump rope. I mean we have the parts for it, right? :P