It´s a great thing if you are very sure of yourself and want to do it. The pros and contras have been discussed, and I have nothing new to offer; just how I would deal with a situation like that..
I could not ever imagine to have a child and not raise it. If any woman really wants to destroy my life from the core, that´s the way to go. I would be destroyed if someone took my child away. I want to have much time, to be a good parent, all of that. I never want to be a weekend father.
Or, in that case, like you say, the "uncle". Imo the whole concept of having a child has nothing to do with uncle-feelings. A parent is responsible, and I want to be responsible for my child.
Shouldn´t a child be born out of love? An aspect that hasn´t been discussed here.
The whole question is if you see the child being "your" child, too. I couldn´t. I also couldn´t donate sperm. It would drive me nuts to know that one or more children exist on this planet, I am their dad, but I don´t know who they are or if I will ever see them. It would drive me insane.
While I surely appreciate the legal possibility for a gay or lesbian pair to "adopt" children, and I am all for it, I could not do it. On the other hand, I don´t know what other possibilities you have to be a dad; also, you seem to feel more free or relaxed about it than me, so if you think about the consequences and you are sure of yourself, why not.
Albeit, a little word of warning: while you trust those wonderful women who are good friends now, things can change a lot in a couple of years.
Consult an attorney, like MK said.