Everyone jumped to the conclusion that he cheated. If this is the case then i agree with their answers. If not, whether this is a polygamous relatioinship or just an open one, I don't think it is selfish of you to ask him to choose. Once a child is involved, many decisions and sacrafices in the realationship will have to be based on that child. Since it is not yours, it is your choice to decide whether you want it to impact your life as well. However, if this is an open or polygamous relationship, if you truly love your husband you will accept his child as well.
Best of luck!