Hey there! Me again! *ducks* Hey, I kinda want to tell my 6 year-old daughter about my sexuality. My wife takes reservation with it for fear that it would get to her parents (my in-laws) and they wouldn't want their daughter married to "a gay". I personally think that if I tell her, she'd grow up more open-minded than what much of society is. Might even be open-minded enough to want to have a same-sex relationship herself later (which I hope to science she does wind up with a woman because as a guy, I can say with relative assuredness that, "guys suck".)
While I like the idea of the open minded growing up, I wouldn't want your daughter to get the idea that daddy is going to run off with some man and leave mommy all alone. She probably won't think that, but you know...children and their imaginations...
Children are amazing and open minded. Your wife is worrying needlessly. They accept things easily and will a full heart. My youngest was 6 when I told her and it was fine. She did out me to the whole school however, and was proud to have a lesbian for a Mom, so be aware of this. Also, what a great oppourtunity to help your daughter understand about Bi people and bust the stereotypes associated with it. She will learn that you love Mom, but if you had loved a guy, you would have been monogmous to him also. That love is love. You could look at this from either way. Also, I would suggest your wife get some counciling for her concerns. It will help both of you. Good luck either way.
Just my opinion, but I don't think a 6-year old understands enough about sex and sexuality to be able to understand anything but "Daddy doesn't love Mommy any more" or a child's version of that. IF you are going to separate, then tell her that in a simple way, but don't get into sexuality yet; she is too young to comprehend what you are saying in a way that will be useful to her later, and there are MANY ways she could misunderstand it at her current age. This might be a good time to seek some short-term help from a child psychologist about the best way to inform her about what the adults in her life are doing.
I think you should be open about it, for the reasons you said. Yossarian has a point though, it might be difficult to explain... maybe "Some men can have girlfriends, some men can have boyfriends, some men can have either... daddy can have either, but he chose to have mummy because he loves her so much". Just an idea, I don't know much about kids so you'll know best if that would work