You don't need a study to prove that a child being brought up by two people in an environment that provides love, safety and mental stimulation is good for them, you just have to look at the children that have been brought up within the system to see it doesn't work without it.
I was brought up by a female. I had no male influence in my life. What difference would it have made if there had been another female on the scene? We learn about relationships from our parents and how two people co-exist. Many couples are dysfunctional. Some children learn that it's okay to mentally and physically abuse your partner or to be scared of one gender.
People don't tend to force their sexuality in front of their children, so apart from natural affection, they would not see the sexual side of the relationship, just the bond and the love between the couple and that is a healthy thing and to learn acceptance of difference is also healthy. You should try it some time