My gf and her husband had their first child about two years after she and I started dating. We came to the conclusion before the baby was born that we couldn't predict exactly what would make the most sense in terms of my role until we'd had a chance to feel out the new, post-baby dynamics, but that in theory I would be an "Aunt" type figure. Things have, indeed, worked out that way. I'm not a co-parent, but I do a lot of babysitting, I love their child very much, and he knows and seems to like me.
We don't live together, for the record.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.