I can't speak for everyone else, but I had children with my ex because I genuinely had no idea what a loving, normal, functional relationship was, never having experienced one. I didn't know how normal people behaved in relationships; and so when I met someone who behaved with extreme abnormality, it felt familiar to me and for a long time, I didn't recognise that it was abnormal.
It was only as a result of having children, that I began to question my own upbringing. Which led me out of denial and made me realise that my own relationship was abnormal and that my XP and I were unwittingly repeating damaging family patterns and role-modelling a crap relationship to our child. And that if I couldn't change it (which I couldn't, because my XP refused to recognise that we were not conducting an emotionally healthy relationship), then I had to leave it, in order to give my children the best possible chance of not experiencing the same disadvantage in adult relationships that I had laboured under.
OK? Will that do you? Is your curiosity satsified now? Or would you like some other LP's to come on and justify to you why they had children with their exes? And when you want to moan about your partner, please bear in mind that no-one is going to ask you to account for why you have had children with him, if he's such a PITA.