Well, things are slowly changing. JLo, Angelina, Kim Kardashian and plenty of other women prove that they can be happy being single. And yes, women without children, single, past 40 or 50 years old, can have fullfilling lives. The best answer you can give people is to live your life and be happy.
As for the people who dare ask what is wrong because you are single, they don't deserve you. In asking this they are proving that there is something wrong in them. They are shortsighted. We do know about childfree people, about introverts, about people who focus on other areas in life. There is nothing wrong in them. They are just living their lives and they owe us nothing.
That is the thing. Life happens and sometimes the best option for us is to remain single. Sure, you could have put up with someone who you don't like just to marry. But that is a terrible idea for both of you.
As for having children, you can adopt or maybe go for artificial insemination. You have choices, it is up to you to decide what feels right for you.
The best defense is to surround with people who love and accept you as you are and ignore the idiots.