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 choose 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.
however, to expand out the pro-choice side of the argument:
it could be argued that it's discriminatory that the mother can choose to terminate a baby, but the father cannot choose to terminate a baby, but the courts still hold each one equally responsible. A good principle here would be to make people responsible for decisions they could make, and not be responsible for decisions that other people made, which they disagreed with.
Also, when a woman gets pregnant she didn't
consent to having a baby because of the act of sex. So she can choose to terminate it, or adopt it out and have no responsibility thereafter. However, equally, when a man has sex he hasn't
consented to having a baby, either. e.g. the "take responsibility" thing is
entirely unfeminist in itself. The guy is in fact under no
obligation to be responsible for a baby because someone got pregnant, any more than the mother is under an
obligation to be responsible for the baby. e.g. any justification here is just as good for the mother or father. e.g. you could say "the guy should take responsibility because he knew the risks when he had sex" but this is in fact no different at all to saying a woman shouldn't be allowed to terminate a baby because
she "knew the risks when she had sex". e.g. that wouldn't fly under pro-choice logic for women, so why should it fly for men?