I've been a single parent. I've been a married parent. And I've been a step-parent to visiting step-children. They are not the same thing at all. Not at all. You can't interchange the situations and make assumptions. Parenting a child who visits is not the same thing as parenting a child you live with. A single mom, earning a living and caring for 2 children, is doing a lot of work. Sometimes parents take shortcuts, like Burger King and pop-tarts. So what?
All of this criticizing your ex's ex isn't really relevant to your situation. Reproduction ability has very little to do with someone's worthiness as a human being. I suggest you forget about the ex and his ex, and work on you and what you want out of life.