I think no matter how a child comes into your home (bio, step kids, adoption, etc) that both selfishness and altruism come into play. Sierras point is well taken, the decision to exclude adoption through the foster system is likely based on selfishness. And that's ok. You can decide how you want to adopt and how you want to parent.
Edit: Also though, please do not presume to know exactly what your county's foster-to-adopt program will or will not allow. Please make an informed decision. Some counties allow foster breastfeeding.