"SDTG: Because contact with biological parents is not essential. Plenty of children grow up happy and healthy despite the fact that their fathers have fucked off and never shown any interest in them. It's a matter of risk, that's all."
Firstly, biology isn't the issue per se - if a person has been in a child's life, having a good, loving nurturing relationship with the them, parenting them, then surely it is better for them to stay in the child's life as much as possible (as long as they are not harming, damaging or abusing the child)?
Why is it better for a child who has not been abused, but who has two loving, caring parents who happen to have stopped loving eachother, to lose contact with one of those parents, biological or otherwise? Is it OK for that child to be collateral damage in an attempt to stop children having to have contact with an abusive parent, however laudable aim is?
And for me, the word 'despite' is key, in your second sentence. Growing up healthy and happy despite their fathers having fucked off implies that the children have had to overcome things, battle through difficulties because their father has left.
If a child does not have to face those extra difficulties and battles, isn't it better if they don't have to face them unless it is necessary? And if the father is a good enough father, not abusive or damaging his kids or wife, why is it necessary for them to have to face these difficulties?