Okay, WHY would it destroy Birthright only? I'm just curious as to why it would. I'm going to play devil's advocate here and argue some points.
Birthright says:
"Select 1 Normal Monster from your graveyard and Special Summon it in face-up Attack Position. When this card is removed from the field, destroy that monster. When that monster is removed from the field, destroy this card."
So with the first line.... no problem there, you've done what it asks to activate it, targetted a Normal Monster in the graveyard.
Here's where the trouble starts. "When this card is removed from the field, destroy that monster. When that monster is removed from the field, destroy this card."
If the monster is Normal summoned again to get its other effect, since the monster is always an effect monster that has the effect of being a Normal monster while on the field and in the graveyard, and it then loses its first effect, of being a normal monster, as was mentioned, and only becomes an effect monster, Birthright would have an illegal target now by that logic... so by the same game logic, since the original target is illegal, something has to happen with Birthright, right?
Birthright WAS equipped to the monster, is it no longer equipped to it, when it loses the effect of being normal? The way I see it, it's still equipped, it's the same monster, it's just so longer normal. If Birthright is destroyed, it's stated right on the card that the monster it was equipped to is destroyed. So the Gemini Monster should be destroyed, as well, by logic. If by some crazy reason, it's no longer the same monster, when it loses the effect of being a Normal Monster, anything equipped on it should be destroyed by that logic, since it's not the same creature.
Does anyone have any sound reasoning why it would work to where only Birthright is destroyed?
I can sort of understand why the Gemini monsters won't be treated as Normal monsters when they've gotten their Gemini effect, as it states then to "have it be treated as an Effect Monster with this effect." I can understand that it may lose its original effect and gain that one, but in the same respect, that should cause a problem with Birthright, if no other card in particular, because of its wording. I guess I'm just looking for the reason Birthright would be destroyed, but not the monster that was equipped to it, in that case, and I'm not finding it anywhere to really make sense.