"Well," said the woman from my choir, "do you have two mommies?" "Yes." "Does she have two mommies?" "Yes... OH!"
That, folks, is how much it took to explain to the daughter of a lesbian couple that it was okay to marry a daughter of another lesbian couple. My choir friend thinks the reason why it was such a difficult concept to grasp is because on television all children ever see is heterosexual couples. TV has so saturated this little girl's world view that she has to ask if something is okay even when it's staring her right in the face.
And that's why Willow and Tara are so cool. There's nothing else like them. A normal couple, both of whom just happen to be women. Even if they have their troubles, which it looks like they will, they will have normal troubles (okay, as normal as you can get when magic is involved) and will work through them the way a normal couple does. Is there anything else on TV that's anything like our girls? If so, I haven't seen it.
Just thought I'd share. 
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Remember the Kitten Board Mantra: "Joss is nuts about Tara, Willow/Tara and Amber!"
To hope is to risk frustration. Therefore, make up your mind to risk frustration. Do not be one of those who, rather than risk failure, never attempts anything. - Thomas Merton
I

See? Nothing to worry about.