I say this with all respect to you and all the knowledge of having been in a similar situation. Plus, I actually have a six month old sleeping right here beside me. Things will change, it is inevitable, it could go any way but I know deep down that I would soon as rip the face off of anyone trying to tell me how to mother my child and so, I think this whole, "you'll be involved too" thing is pie in the sky wish fulfilment, since they are not yet parents, they just don't know.
If anything, it would only be on the periphery, like fetching for her or holding the baby so she can have a bath, not parenting because women are very loath to give up any of that even to the fathers, you know newborns are very time consuming and women are pretty crazy the first few months (I consider myself a prime example).
If you consider riding it out and staying, you will feel like the 3rd (4th?) wheel, that is also inevitable, but if you have to really ask yourself how integrated you really feel, you say they asked you to move in once but you said no, but if they asked again you would do it in a heartbeat.
Why the heck do you need to wait for them to ask? If you don't feel comfortable saying 'I think we should take the next step and move in together, what do you think?' than how can you really question whether you are really equal? Because you aren't, not really and I think lots of us who have been there know this deep down, but we don't want to believe it.
Also, it seems they have been together for a loooong time, since childhood or, is she getting on? If she is getting on a bit, she might find it hard getting pregnant at all. Don't think for a minute you have a chance of having your own baby if she is having trouble getting hers. In fact that idea might be vetoed even if she does get pregnant since many women are very protective of genetic rights to their husbands DNA.
Enjoy the relationship for as long as you can bear it, but I don't think it is possible to really feel at ease in that situation, but that is me, YMMV.