My first relationship at 16 resulted in my eldest DS 18 months in. We stayed together for 8 years. In total. He lost his job at the 7 year point and over the next year or so I lost all respect for him for both the reason he lost his job, and for his lack of effort in sorting out a job or retraining. There was just no coming back from it, it gave me the ick. It was doomed from then on, but also, the fact that I'd spent the previous 8 years being a teen mum, then going to uni, buying us a house, teaching him to drive and buying him a car. Basically making sure our child's future was secure. I'd changed, he had not. He was never an immature kid, but he never sought to do better in life. He didn't want to expand his knowledge on any subject at all. Which is fine. But it wasn't what I wanted in a partner. I'd have been unhappy with him all my life if I hadn't left him.
Next one cheated. No explanation needed. That was the father to my younger 2 children. He buggered off completely when I was 7 weeks pregnant with my youngest. Good riddance to bad rubbish.
I then met someone who I dated for 8 months. Introduced to my young children around 6 months. He had a very young child too, 10 months old. Mine were 16 months and 3 (oldest was a teen by then). The moment he pushed away (with zero care as to where he might land) my toddling 16 month old who was a little unsteady near his precious 10 month old, I called it quits. He had already shown signs that being a stepdad may not be for him. He was on course to be Disney dad, despite having his daughter 3 nights every week too. I put my kids first, and called time. I'm glad I didn't wait longer to introduce them, or it would have been much harder to walk away.
Current partner I've been with 4 years, and I hope that there won't be a reason to call time! He is lovely, intelligent, kind, great step dad, not a Disney dad to his own(!), and all round good person.