Parents are parents. Straight parents are parents. Gay parents are parents. There's no fundamental difference.
If nobody had such beliefs, then it is virtually impossible to understand why we would suppose that gay people are unfit as parents.
While I don't agree with these arguments (for the record, I'm gay), it would be possible to make them without being homophobic. Would they be made in nobody had the beliefs you describe? Perhaps not, but they could be.
Just no being a drug-addicted and not having a debilitating mental disorder is enough to be an above-average parent.