An argument for continuing with the practice of anonymous sperm donation I really dislike is "So many children are born without knowing their fathers for various reasons beyond our control. Therefore, there is no right to know one's father, because you can't always enforce it."
First, that's like saying there's no right to private property because you can't always enforce it, as there are thieves in this world who never get caught by the police.
Second, google "I never knew my father" or something to that effect and you'll find oodles of heartbreaking stories by people whose right to know their father just wasn't enforceable. And these are mostly not DC adults or even people who had abusive stepfathers - just human beings who were never allowed the chance to meet their biological fathers and who are still suffering. Some of them fear they don't have the right to their grief, and some even apologize for it, but they still can't help feeling it.
Third, yes, injustices happen in this world - but do we really want to defend doing this to children intentionally by noting that it happens? That would be like parents wishing to genetically engineer a blind child (they themselves can see, but they really really want a blind child) and justifying it by saying that so many children are born blind every day and many are blinded in accidents beyond our control, and that particular child wouldn't even exist if they weren't allowed to engineer it. So the child could only be grateful, right?
Everyone knows so many blind people and they're all just fine, really. We know they're fine because they don't say to everyone around them "You know what? Being blind sucks. Life is a struggle every day. I don't think my life is any less worthy than anyone else's, and I certainly don't wish my existence away, but being blind is hard."