Yeah, I know the feeling. :-)
You can't be bitter about it. It will just eat you up inside.
Because I suffered from endometriosis, I was started on Depo Provera shots, which is a form of birth control, and the knowledge that I wasn't pregnant every month became a Godsend to me. We have come to the realization, that for whatever reason, it was a blessing that we had one child, and our one child is a true miracle. And we have the happiest marriage out of anyone I know. My husband is my best friend, and looking back I am grateful for the life I had. I totally understand your feelings of people assuming that everyone is childless by choice. I remember having mere acquaintances asking my young son "When are your Mommy and Daddy gonna give you a brother or sister" and my heart breaking. It was so hard not to be rude to them. I finally got to the point of saying "I guess some things aren't in Gods plans". At that point I made a vow to never, ever ask a child free couple, "So when are you going to start your family".