I'm having trouble parsing what you just wrote and how it's supposed to be logically equivalent to what Lord Shonus wrote.
If men were all sperm donors and first saw their children at age 18, then men would have no legal custody of their children
Well people who are donors don't have custody.
Should that mean that the mothers, who keep the children, shouldn't have legal custody of their children either?
Well no, but women who give their children up for adoption don't have custody. You're comparing apples and oranges here of people who do not keep the child vs those who do then assuming some fictional fantasy world that assumes all men are sperm donors and all women are caretakers, then saying that sperm donors and caretakers must have equal rights, so as not to be gender discriminatory. There are too many logical sleights of hand pulled there.
What Lord Shonus is saying is totally different. He's saying that the argument goes that "males do not get to choose to kill a baby so therefore females shouldn't get to kill a baby". I'm pro-choice but I'm having trouble seeing how your analogy fits that.
e.g. if you forced all men to be sperm donors only, and disallowed them custody, it would in fact be discriminatory to not also make that ruling for women, therefore in that case women should not be awarded custody, and the children would need to be provided for by the state. e.g. the sleight of hand there is that you mixed up "sperm donor" which is a voluntary thing with the idea of forcibly taking women's children away, which is an involuntary thing. It's discriminatory if either gender is subject to a forced ruling that the other is not. It's not discriminatory if both are forced to abide by the same ruling, no matter how shitty that ruling is.
You're right, that was a poor analogy given that custody is not determined by genetics. My thought experiment was that men were sperm donors by
custom, though. (So it technically stands, but it's a stretch to say that it proves anything about the original argument.)
Perhaps a less directly relevant analogy would be better? “The law, in its majestic equality, forbids rich and poor alike to sleep under bridges, beg in the streets or steal bread.” Except not quite, because it's not that men have no need to have an abortion, it's that they're
physically incapable of having an abortion (barring the odd trans/intersex man).
Or maybe "it's not discriminatory against gay people to ban gay marriage, because both straight and gay people have the same right to marry the opposite sex." Caveat as above, but I think the analogy still holds.
My argument itself is that the original argument presented (although not necessarily
endorsed) by Lord Shonus is engaging in sleight-of-hand. Everybody is forbidden from killing babies who have been born. Men can only access babies after they have been born. Thus, in practice, if any man has killed any baby, then that is illegal. But this is not itself a law - it is emergent out of the law against killing
living babies, and the fact that unborn babies are inaccessible to men. If you take the emergent restriction as a law and then generalize it, then that's technically Fair and Objective, but it's also Fair and Objective to say "only kill babies who haven't been born" which in practice only allows women to kill babies.
Except I feel like I've reversed something. I think this argument went wrong somewhere, but I can't tell where.
I've got it! This is a Fully General Argument. All it shows is that it's more complicated - it can't legitimately support a specific side. But I already dealt with this when I said that both ways are Fair and Objective, so it's pre-salvaged.