I guess the main problem is that pregnancy is considered a health insurance thing at all: it's going to come and birth, like schools, benefits society as a whole. But who pays for it?
If you are a single guy with no kids, do you also complain about property taxes funding public schools? After all you have no kids, you are paying for something you don't use. If you go single or DINK, then it is something you will never use.
Really, this is why I think single payer or just full-on nationalized health would work better in the end. Insurance is just another tax now, and there is no point to put private companies in the middle of it.
Prenatal care like vaccinations more than pay for themselves at the societal level. I can see the argument for keeping the government out of healthcare, but this really does fall under moral hazard.
Also, if a female can go from no prenatal care to prenatal care then there going to swap from one to the other when they decide to have kids which would distort the market. In the end it's the same reason you can't get insurance with and without support for specific diseases.
Unintended pregnancies exist, and people have sex. But you're not going to get pulled over and cited for sexing without a license, because it tends to happen in private. So how should an insurance company verify that someone isn't having sex that could result in an immediate need for prenatal care? What happens when you're allowed to opt out, but your contraceptive fails and you have a kid anyway? The risk of ectopic pregnancies and/or miscarriages is high pretty early on, and many of those present in the ER where it's ludicrously expensive to even breathe the air.
How about diabetes or mental health care, should you be allowed to opt out of that coverage it you're sure™ it's not going to happen to you? Can women opt out of most everything to do with the prostate?
On top of all of this, as another commenter explained, the marginal cost of prenatal/pregnancy/postnatal coverage isn't even all that high. The cost of managing the opt-out would probably destroy any savings you were hoping to realize.
