You are not a bad person because you don't want children. As another poster (or two!) has mentioned, you are displaying good judgement. If you don't think that parenting is for you, then it isn't - period.
I see a lot of posters here have mentioned that you may change your mind over time, and you may. But then again, you may not. I'm almost 48 years old, and I have never wanted children. I have never felt that my clock is ticking away and that I'd better hurry up and have kids while I still have time. I still don't want kids. Even my gynecologist keeps asking me why I haven't had children.
It seems that lots of families put pressure on women to produce children. My sister still tells me that I haven't stepped up and taken on the responsibilities that I should have because I didn't have children. Apparently, she thinks that all women have a responsibility to have children. I find this a bit amusing, since I was with her when she delivered her second child - her exact words to myself and my mother were, "I only had this kid because my husband wanted one." They have since divorced (surprise!) and she's working on her fourth marriage. But - she accepted her responsiblity, I guess.
It's not for me, and if it's not for you, don't worry about it. You'll have to develop a thick skin, though, to let all the remarks go by. It's better to just let it go by than to argue about it with your in-laws.
You'll also find that at work, those with children get preference over those who don't. I've experienced this at every place I've ever worked. Apparently, those of us who choose not to have children are automatically assumed to have no other life, since children are apparently the only thing that matters. It's discriminatory, but it happens - people don't even realize they are doing it. When days off are scheduled, those with kids almost always get preference over those who don't have kids. Parents are expected to stay at home when their kids need them, and those who don't have kids don't really have any special reason to need time off.