this isn't an excuse for them... I don't understand it either, other than some men don't get in the caretaker/giver role and step up to their responsibility. I have been married twice, both time as the 2nd wife to men that had families. Also had a sort of long term relationship (getting smarter, didn't marry that time) to a guy that had kids. All of these men, one a dry drunk, the others active A, didn't take responsibility, nor care about their children. 1st exah, didn't see his d from the age of 4 till she was 14, and that was only when he was pursued to pay his child support... 2nd - dry drunk, religuished his parent visitation, so he Didn't have to pay his back child support and the xabf, I would guess - still owes thousands for this adult children - they are in their mid to late 30's. It's probably a good thing, these kids had at least one reliable parent, who cared...
Take care of you and your kids enjoy them all you can.