My first LTR was like that. She had a young son from her first relationship to whom I became a second dad for about 15 years (duration of that relationship during which my three now adult kits came to be). Never married though.
That 'young son' is now 27, with two kids, works full time in the army, and lives in a different state.
My second ex has no other kids besides our 6 y/o daughter and when she was born I was 48 and she was 42. Again never married. But because our daughter is IVF + egg donor (biologically mine, not biologically her 'mums'), she has a bigger 'family' (from which I'm basically excluded) with the actual egg-donor parent's kids and another egg donation recipient's kids.