Even if there was compelling evidence that homosexuals in general are worse parents, and even if that evidence was that they were on average substantially worse parents, no average evidence is a sufficient reason to take away a right or treat someone as anything besides an individual. Of course, such evidence does not exist: The overwhelming bulk of the evidence indicates that homosexual parents are just as fit as straight parents, and even the indicators to the contrary routinely are clear that any barriers that are faced are due to the effects of homophobia such as stigma and institutional disadvantage. The research critical of gay parents has routinely been incredibly dishonest and hobbled with clearly intentional methodological flaws, and that research has been heavily funded by groups like Heritage, the Witherspoon Institute, and so forth.
The only factors that should dictate if someone is allowed to adopt a child are individual factors that relate to their fitness for childrearing: income, past criminal record, etc. If gay parents are as bad of parents as conservatives allege on average, then each individual bad parent should of course be denied, but those that would buck that trend should still be allowed their rights.
It is a tragic indicator of how deeply ingrained our tendencies are to not view rights as sacred and as the default state to only be threatened when there is an immensely compelling reason to do so that so many conservatives, including small-government conservatives, have rushed to deny others their rights based on who they choose to love and associate with.