If I recall correctly, Obamacare mandates that every insurance plan must include certain things, like pregnancy coverage, whether you want them or not.
Just remember that when someone tells you that the USA is a free society.
I think it's illegal because insurance companies don't offer boy policies or girl policies. So yes, for men you have to buy policies that cover pregnancy and birth control. But by the same token I have to buy policies that cover prostate cancer and Viagra.
My assumption is that there are no cafeteria type polices for the same reason why there aren't sex-specific policies; it'd be a nightmare for the insurance companies to manage and to price. It's much easier for them to offer blanket coverage and average out the cost.