It is a weak argument.
Logic is not the strong suit of supporters of abortion or liberals in general.
If he causes the abortion, it's murder. If she causes the abortion it's just a lifestyle choice.
Inconsistent application of the law is one of the things that breeds contempt for same.
Personally, I think it's murder in either case.
The biggest problem with prosecuting this kind of crime IMO, is the there is such a thing as 'spontaneous abortion', which is essentially an act of God. This case would be easily provable not to be. There are edge cases though, where that is not the case. That's the only problem I have with making abortion a criminal act. If it were ever made so, I'd hate to see innocent people prosecuted for an act of God.
It's important to remember that abortion WAS illegal and WAS prosecuted in all 50 states before Roe vs Wade. Fifty out of fifty.
But the type of difficulty you describe --- false prosecution because of a miscarriage --- never happened in the legal history of abortion law.
This was because abortion was more-or-less treated as a 2-victim crime --- the woman and the doctor --- and there were not prosecutions of the doctor, generally, unless he had put a woman in the ER with a ripped cervix or a lacerated uterus.
Keep in mind, too, that the death rate for human beings is 100%, and most of these, too, are "acts of God" : illness or injury. But deaths are generally not criminally investigated unless there is clear evidence of foul play.
As you mentioned, in this case of a guy inserting Cytotex into the woman's vagina, it couldn't be clearer.
If I were judge and jury, that guy wouldn't walk free until Christ comes again. And then he'd face the real judgment.