That's a self-defeating argument: if this were registration of Clans, it would be valid, but as this is registration of - Western Style - House names, unfortunately it is not.
The point Moot is trying to get across is that we are using a specific naming system and we're asking people to stick with it for both ease of understanding and because most people will easily understand this system. As you can see, the system you're asking to use is somewhat more complicated.
If we make an exception for this we'd have to make an exception for anyone, I believe is Mootles' chief argument. For convenience, can you not simply accept the word of the Founder?
EDIT for clarity: We'll be using House names in the same way as most people understand them - if you 'marry' someone from House Avestrii, you become a member of that House and take their last name. Should two individuals from separate Houses marry, they'll decide which name to take and the other will become part of that House, merely referred to as a Scion of their original House, and not as an actual member - they've married
out of the Family.
Taking a House name was a big deal in medieval Europe and not something undertaken lightly. If we're using the system I think we are, your argument is unfortunately invalid. :/