Congrats on having a new baby. It sounds like the baby will have lots of positive adult attention growing up.
As for coming out, I think you will have to come out big (full story) instead of small (bits and pieces dropped every now and then). I have a feeling you will get many different types of reactions. For the negative ones, one response may be that they see you dealing with the consequences of an unnatural, sexual lifestyle. Another respsponse is concern that this will ruin your marriage or hurt the kid.
I think you should address that everyone is committed to helping the kid grow up and that there is a loving environment.
I think this will greatly challenge many people's views of what a family should be. So just be ready for anything. Know that some reactions will be gut responses with little thought. Allow for time and show patience. Some people will not be convinced that this is ok until they see it in action.
I think the worst you may see is people forbiding your whole family from visiting since they may see it as wrong. You have to decide if you will only bring some people or refrain from visiting entirely.
When I was in a triad, I let people know that both women were my girlfriends. If we stayed the night somewhere, we would stay together. A lot of times that meant that we did not spend the night somewhere.
I wish you luck.