Noone has a right to adopt, and nor should they. The only relevant criterion is "Is this person going to be able to provide what this child needs?"

Of course that just throws the question back to the larger one of who decides what a child needs, and judges whether this person can provide it - but I can't see that having rules of thumb about categories of potential adopters really makes much sense. There is no escaping the need to make judgements on an individual basis. After all, there are plenty of married couples who should never be trusted with a child.