Gay people tend to adopt (and may adopt out of foster care), which means that many of these disadvantages may be in place before the kids are ever in the gay household. For example, maybe these kids spent time in foster care before they were adopted by gay couples. Maybe they were abused by their biological parents before being put in foster care.
A better study would compare kids raised by gay couples to adopted kids raised by straigh couples.
That was my thought as well. The study should try to factor out the adoption aspect, or at least make steps to equalize the number of bio vs adopted children in the study. Otherwise, you have an indictment of any adoption.