The pro's of proving scientifically there's a physical difference between the brains of hetrosexual and homosexuals are:
The first is obvious, we will know that it is not a matter of choice because very few if any humans or animals can consciously control the development of their neuropaths. This would then make it harder for people to justify homophobia.
Secondly, it would then give us a bases to research the causes and ultimately to see if it is genetical or not. Thereby, reinforcing or not the case of if it's choice or not.
Additionally, if anyone thought they were homosexual they could be “tested” for it. The risk of testing would me that it makes it more of an “illness” but if it was approached in the same way as intelligence with the IQ test then maybe not. This however leads away from the neurology and genetics and into the field of psychology. With many things in psychology they aren't solidly defined and the “choice” argument could creep in. I doubt though, that anyone would “choose” to be clinically retarded nor would they choose to be in a minority (homosexual). Yes, homosexuality could be seen as a negative thing still but at least there would be a marked and defined difference in thought processes and such. How practical this would be I don't know.
The downsides are:
That if there is not actually a difference it will reinforce the choice opinion and help justify homophobia.
However, if there is a difference that isn't genetical and is due to the way homosexuals are brought up. It could lead to abuse of children, young adults and adults that take solace in religions because they find it difficult to understand their feelings.
The people that come out when they are older may then feel worse for not knowing who they were before.