Counterpoint
Even if it were true, that the ideal environment for a child is a mother and father, which studies show it isn't, that still wouldn't justify a flat-out ban. Most governments still allow single people to apply for adoption, and even single gay people1. That is because there won't be an 'ideal' family available for every child who needs a home. So other options should be considered. After all, a child is better off with 'non-ideal' parents than with no parents at all. With adoptions, there is generally great demand for babies and toddlers, but older children are generally unwanted2 and end up in foster care until they're 18.
Proposition fails to tell us what studies they are referring to which does leave the question open whether these studies have taken into account other factors such as whether or not the biological parents were drug users. The heritage left by the biological parents needs to be remembered.