The logic that a fetus is a person comes from the inevitability of birth starting at conception — as long as no one interferes, a human person will eventually be born. Therefore, ending the pregnancy early means that person will never be born, so it’s tantamount to killing a newborn baby.

Once you accept a fetus is a person, this same logic can also be extended to argue that life starts at ejaculation; impeding sperm from fertilizing an egg prevents the birth of a person, and is therefore tantamount to murder the same way abortion is.

It’s much more a tenuous an argument, which is why it’s only made by the most extreme people. But I guess that explains why Clarence Thomas is pushing to make contraception illegal today.