Your hypothetical NAMBLA bill raises some major issues regarding the age of consent, which complicates things significantly. So it's a pretty lousy comparison.
As for normalcy, gays are clearly not normal in the same sense that red hair is clearly not normal: only a small percentage of the population possesses the trait, and "normal" is generally based on the majority. On the other hand, unlike red hair, as best we can tell most every culture in history that hasn't actively persecuted gays (and thus driven them into secrecy and out of the historical record) has shown roughly the same percentage of gay members, so it would appear that having a certain percentage of gays in the population is normal for our species. The question is simply should that percentage of the population be denied the right to get the same legal recognition for their long-term relationships as we have for ours.
They're not asking us to celebrate anything. They're not demanding that your pastor perform the ceremony. They just want to be able to go down to the county courthouse and establish the same legal rights of inheritance, medical visitation, etc. than heterosexual couples enjoy. I don't see how them doing so harms society in any way, and so I see no rationale by which society can claim a legitimate interest in denying them such rights. Maybe if being gay was contagious then you would have some good arguments for suppressing it's spread, but it's not.