Reluctant mom..kudos to you for wanting to open your home to another child. I find it quite funny how ( as you say) when you have children "naturally" no one interrigates you as to why, but when you want to adopt suddenly you're called upon to justify your decision. Although I don't have personal experience with adoption, my family is a fairly complicated mish-mash of relatives who have been raised by people other than their parents ( and call their adoptive parents mom and dad as opposed to aunt and uncle) and I have a half brother and step brother - one of whom is very light ( coloured-looking) the other is very dark ( Kenyan parents). Add to that the fact that there are white people in my family too so I have had experiences where people ask if I am my niece's nanny!
My point: new kids always upset the family dynamic - but there are enough "step-families" out there to prove that biology only counts as much as you make it count.
Secondly, who gives a **** what everyone else thinks? this is about you and your family. And as for your kids, take their views into account, but no more so than if you were considering getting pregnant again. It makes no sense to me that you would allow them to dictate a decision like this when they will eventually be out of the house with their own lives to lead