All I have to say is that when the baby comes, things will be OMG DIFFERENT. If you think you're in a "secondary" position/role NOW, don't expect to be up-graded when the baby comes.
I know it's easy for me to say from where I sit, but I think you should let them go and do their baby-thing together and cut your losses and find someone(s) who are able to give you what YOU need out of a relationship. I'm probably gonna get shot down for saying this: I don't think you will find what you say you want by "joining" an existing couple (unless it's a couple that is very experienced with having a "third" AND is adept in dealing with the issues that come up in such a situation).
THis whole thing with couples and "thirds" freaks me out a bit because it seems that often the couple is viewed by themselves AND the prospective "third" as "one unit", when they are NOT, no matter how you try. They are TWO PEOPLE with TWO SEPARATE sets of needs, TWO SEPARATE personalities, TWO SEPARATE sets of expectations, etc., and nothing anyone can say or do will change that.