My father is the oldest of 8 children, but grew up with the next two down from him (his full-siblings). His father, my grandfather, didn't lay eyes upon my father or his brothers from the time Dad was 5 years old - yet he knew where they lived, as he used to write to my grandmother every now and then (never mentioned their children, though; only the ones he had with "the other woman"). The younger children all knew about Dad and his brothers - apparently as the eldest son, my father was held up as something of a shining example... by a man who simply did not know him at all.
Do I judge my step-grandmothers (there were three more marriages) for having more children with my grandfather, even though he didn't see his elder three children? Yes. Do I judge my grandfather for essentially abandoning my father and his full-siblings? Undoubtedly. He'd died by the time I traced his family, and I've listened to so many "oh, he was such a great man!" stories that it's not funny - because I don't think that he was.
My father and his full-siblings refuse to have a relationship with their younger half-siblings, nephews and nieces... which is their right. The hurt they all felt as children, and feel as adults, is too much for a successful relationship structure. But I have no memories of my grandfather, and actually? Quite like his "other" children and grandchildren/great-grandchildren. We talk often, and I've learned a lot about the man my grandfather was to them. The irony of it is that he was apparently desperate for a granddaughter, but had only grandsons... all of whom are younger than me. The eldest grandchild and the granddaughter he was "desperate" for. If he'd maintained contact with my father and his brothers, he would have had 3 granddaughters alongside grandsons, but he decided he didn't want to be in his older children's lives - so he missed out. His loss. Not ours.
My friends who have divorced or separated from their children's mothers, on the other hand, have maintained a steady and amicable contact. My oldest's father? To the best of my knowledge hasn't set eyes on her since she was 3 weeks old - his choice; despite his abusive behaviours, my daughter saw his family regularly until she was 7 years old. Just not him. He went on to have two more girls with someone else. My son's father's contact is still very focused on the adult in the relationship. If he decides to go on holiday with his other children, our son is relegated to the back benches, for example, where as on other weeks, my quality time with our son is thrown to one side because my ex has clicked his fingers and demanded that our son spend time with him (which is frustrating as fuck!). Some men are simply thoughtless and arrogant enough to believe that it's all about them.
(Neither my grandfather or either of my exes pay/paid maintenance, either... which is somewhat telling as to the sort of "fathers" they actually are/were).