I also think it's really OK not to have children if you don't want to.
Part of my original plan was to adopt, but then I got pregnant and my husband and I wanted our baby (not a baby anymore, he's 4yo). He is a ray of sunshine in our home, and because of the questions you mentioned, he gives me an opportunity to reevaluate the way I view the world every day.
I (almost) always think carefully before answering any question.
For instance, we are not going to send him to any religious school, and he was not baptized despite my husband's family's pressure. We intend to show him what we perceive as good and bad aspects of the religions we know something about, and then let him choose. I love my son very much, and I think my job is to nurture and protect him, to teach him to think critically and grant him access (as far as I can) to education and information, and try to help him with the means to become what he chooses. I hope we'll get it right
It is a huge responsibility to put someone on this world!
Don't let society (or anyone) pressure you to have children. Lot of people are already having them
There are many ways to be happy, and many ways to contribute to the world.