Its not whether they have the right to be parents, it is whether their rights impede another's right. In this case their rights to be parents impede the child's right to live without fear in safety. If they are people who would make decent parents they would not put their future child's happiness and safety at risk.
Sure if we were a better society and the children are not victimised because their parents are the same-sex then go for it! But that is not the case and I have seen way too many children suffer because of same-sex relationships. And, I know that most of those parents regret what they put their children through. It is not fair but this is not a fair world.