Well, I haven't had this problem (I'm a bmom with no other children), however, it's not your father's decision, is it? It is too bad he holds these opinions, but I wouldn't let it stand in my way if I wanted to adopt. If his attitudes spill over onto his relationship with any children you decide to adopt, I would probably limit the time they spent around him, at the least. His statements are false. Not all adopted children have medical issues and even if you had bio kids, there is no health guarantee that comes with them, either! As far as bio kids being harder to raise, well, heck if that were the case, wouldn't more people adopt?
I mean, why would he want your life to be
harder?? That comment just doesn't make sense to me.
Honestly, if you wish to adopt, you should. I would simply tell people (father included) that you've made your decision and it is not up for discussion. If he continues to spout off negative views, keep telling him it's not up for comment and if he doesn't stop, you will have to leave/end the conversation/go home/whatever (fill in the blank). Lather. Rinse. Repeat.