I don't think that holds water. Gay rights are great, but childrens rights are greater. If there is any doubt about the rights of either group being infringed on by 'experimenting' then you have an immediate problem because nobody will sign off on experimenting on children to see if in the long term it will matter or not.
If there would be large numbers of cases proving that it essentially makes no difference then that would help, but you're in a chicken-and-the-egg situation.
I know of one gay couple in my circle of friends that would like to adopt a child and they've come to the conclusion that they themselves can't come to a 100% agreement between the two of them about what that responsibility exactly entails. It was a very eye opening discussion, because I think I went away a changed person, essentially form going to saying 'I don't see why not' to 'maybe it really is better, who knows'.
For me that's a weird thing because when I thought about this before I always thought that gay people would not 'overlap' with straight people on this issue but would be on the far side of a barrier that you can not cross because you can not imagine what it is exactly like to be gay when you are not.
This is a very complex issue, societal pressures are such that simply being raised as the child of two gay parents puts undue pressure on a child (for instance peer pressure), and that can cause serious damage. That's the sort of argument that they went on about and I had never even thought of it from that angle.