I should have kept the original title for this piece and prefaced it to set a perspective. the original title was "On the subject of gay marriage and constitutional amendment" and it was written in response to an attempt to amend the U.S. Constitution. In that context I feel that citizens of the "age of majority" would have been presumed by the reader. I sadly must agree with you that my first point is not the "status quo". But I speak to the ideals of the document. The Constitution already provides for the ideal of equality, It is "We the People" that fall short in making it the truth.
Indeed we do agree that the government has no business being involved in "marriage" by my definition of it. The civil union does however exist, regardless of it being an undesirable state of affairs in its current form. That is why it must be changed if not abolished.
Indeed they would, for many it would close off the path to citizenship that is offered through the civil union, and for even that one reason, though there are many more, the concept of civil union must be corrected rather than abolished.
No, I do not believe that civil unions should be restricted in any way. If (using your example) Frasier Crane and his father decided to enter a civil union, merge their assets and give each other specific legal rights in regards to those assets. Yes they should be able to form a union. As for your three member couple in England. In the context of this post, No. Only because the U.S. government has no authority over citizens of England. If that trio were U.S. citizens then yes they should be able to form a union. I would also add that the union be able to be amended at a later date to include additional partners should the original parties choose to do so.
Again this is just my opinion. Your mileage may vary.