Quath, its funny you say that. Back when I was learning it was ok to love multiple people, I remember thinking about the same thing, and I have talked to her about how love works with kids, other family, friends. Her next leap was to say the only thing left is sex.
The key for us is to talk with you guys a lot and take it slow. I am taking the first step soon, but that is not to say I am going to be running quickly into anything. My wife knows the girl, she had babysat our son several times. Shes known I liked her because I have told her before.
W (wife) is struggling to see how things can work, its almost as if I am not seeing somebody, or the moment I say hello they will suddenly be on the same relationship level as her and I. All relationships take time to develop. So we will just go one small step at a time.