Six years ago I discovered my ex's affair, which ended our twenty year marriage. He effectively abandoned his teenage kids to be with her and her teenage boys (she had left her husband to be with mine).
My son, now twenty, has never forgiven him this. He sees his dad perhaps six times a year, although he only lives ten miles away. The meet ups are strained and difficult. His older kids have a better relationship with him, although not great by any means.
What astounded us all was when he left, ex really thought his kids would be fine. Loved up and blissfully happy in his new relationship, he assumed they would be delighted to have a happy dad. He couldn't see beyond his own needs.
Well he could, but he wouldn't, and that is the crux of this OP.
To dig deep and admit that you have changed the lives of the kids you fathered takes guts. It means that you have to face reality. So they don't, that's too painful and raw. So they minimise the pain they cause to make themselves feel entitled to live in their fantasy. They deserve this, they convince themselves, it is their right. Their kids should be happy for them, they think. But those kids aren't in a fantasy, they're in a horrible reality that they didn't want, cause or deserve. Often trying to cope with the parent who feels they've been thrown under a bus.
My ex is not a happy man, because the heady fantasy days with the woman he gave everything up for are over. They both now realise what they lost, five adult kids between them, who are ok with it, but resent them for it.
My ex, the most self absorbed entitled man I've ever met, blames me for the fractured relationship with his son, because he isn't quite able to own his selfish behaviour and the way he abandoned his kids. Sometimes the cracks show, he gets terribly upset at the way things have turned out and I'm sure he has regrets. Crippled with guilt, he is unable to really reach out and talk and rebuild the father and son bond.
Fathers (and mothers) who leave a family are not ogres, but they should accept that they need to move heaven and earth to maintain close relationships with their kids, however old the kids are. Frequently they don't, because if they did, they would need to own up to the heartbreak they've caused. So they take the easier option, and immerse themselves in their new lives. Then years later, they start to pay the price. It's very sad, but all we can do is support our kids, encourage a relationship with the parent who has moved on, whilst accepting that the onus for the relationship lies with the one who left.