Hey, i just came across your post.
You have absolutely no reason to feel guilty for wanting your own children. My mother was unable to conceive again after me (she was remarried). She and my stepfather tried various forms of infertility treatment for almost 2 years, but eventually had to accept that they would not have another child biologically. They became foster parents and have since adopted three children. We had all of them from birth... picked them up at the hospital as a matter of fact.
It doesn't matter how you came to choose adoption or that it wasn't your first choice. What matters is that you're considering it at all. I've been around the system for a long time and i know what they go through. The number of children needing homes is staggering. And do not be put off by the misconception that it takes years to get a baby (if you want a baby). My parents picked up their first baby just 4 months after becoming foster parents. Granted she was premature and drug addicted now she is a beautiful and extremely intelligent girl. If you keep your heart open you'll find the perfect child for you.
I'm sorry this is so long. i just feel the need to help people who are going through (or considering going through) the process of adoption. I'd be glad to try to answer any questions you might have btw.