Thought it was pretty clear. Real-world biology isn't binary.
Someone who has extra (or missing) chromosomes isn't less human and doesn't deserve to be treated as such. Or someone whose brain is put together a little differently even if they don't have an easily visible genetic difference.
Was Einstein "abnormal"? Was Turning? Was Meitner?
Insisting on putting people in bins is stupid.
Your personal feelings about "the purpose of marriage" and the "definition of a family" should have little or no impact on whether two others want to get married or not. Or whether they get the benefits of marriage and family that other couples get, or not.
But, as you say, we've been through all of this before.
My $0.02.
Cheers,
Scott.