I think men generally prefer biological children if they are fertile, we are wired that way.
And I agree that it's normal for men to fear/worry that they won't love their adopted child.
But when a couple agrees on adoption and finally get the child, the parenting switch is triggered and the responsibility dawns, then men will love their adopted children. I have seen that with my aunt and uncle.
I can see why you are having a hard time finding a guy that prefers to adopt, but I am sure he is out there.