I can see a very positive effect on a child raised in a household with two nurturing parents, both of whom have open minds. I believe that a good parent begins from positive experiences and positive interpretation from negative experiences. A human being is very much garbage in/garbage out, we can grow in leaps and bounds as a person from overcoming hardships.
In regards to the results of this particular study, it may be that once one has come to terms with their own sexuality and in this case the social stigma associated with being homosexual that it can open ones view quite a bit wider than say your average heterosexual. In this line of thinking such an individual I would think possesses a greater capacity to be accepting of others, would seem to lend itself rather well to child rearing. Add in the fact that in this particular study that each child was planned, the parents or sometimes parent was ready to accept the responsibility of bringing another human being into this world. Starting off on the right foot can make ones journey much easier, not to say that accidents cannot spark a wonderful journey on their own, just that knowing what path you are taking makes it easier to prepare for your responsibility.
Overall thank you for bringing the article to our attention Indigo, this fellow found it to be a very interesting read.