DMcG: But to flesh that out, there is no more a scientific reason for or against gay marriage than there is a musical one. It is a completely irrelevant mechanism for deciding the issue.

The reason it's relevant is that two of the main reasons given for denying civil rights to homosexuals are that they have chosen to live that way and that their way of living is perverted. The scientific evidence presented by these groups shows that it is not a choice and that it is normal.