With regard to "twin studies" ...
There are, I believe, a couple hundred identical twin who were separated at birth. These people have been studied to the point of exhaustion. But, given the low incidence of homosexuality, these twins don't constitute a large enough sample to say anything definitively about genetics versus environment.
Looking at fraternal twins separated at birth is the same thing as looking at siblings separated at birth. You can get a larger, and statistically meaningful sample, using fraternal twins or siblings separated at birth, at the cost, obviously, of inexact genetic matching.
Leaving aside the problems with "separated at birth" studies, studies of fraternal twins/siblings who are separated at birth tell us that genetics influence sexual orientation, but are far from controlling, which means that the results of these studies are rejected out of hand by those insisting, alternately, that homosexuality is either 100% or 0% genetic-based.
The argument that homosexuality is 100% genetic-based is about a lot more than the sinfulness of homosexuality. If homosexuality is less than 100% genetic-based, then gay-adoption is problematic.
To the extent that we influence the adoption decision, we would want children to be raised by both a male and a female role model, preferably (1) their birth-parents, or (2) close relatives such as an aunt or uncle in a traditional marriage (as traditionally was provided by naming such people as the god-parents of your children), or (3) by a non-related couple in a traditional marriage or a single close relative such as an aunt or an uncle.
Only after exhausting the above possibilities, should you consider adoption by a single non-relative to raising the child in a group home with male and female role models provided "by committee."
In terms of the culture war, this is what elections are all about (which is not to diminish the importance of free-market economics and a government with the strength and resolve needed to defend us against criminals and foreign enemies).
Gay adoption is problematic period... Recent study from the netherlands basically proved this one, confirmed what all thinking and honest people know.
In the netherlands there is no social stigma to being homosexual, or to children raised by them, yet children raised by such couples were found to be at much higher risk for all sorts of problems.
Even if they could (they can't) but even if they could prove a 100% genetic link to homosexuality, homosexual adoption is dangerous and hazardous to children.