Beyond which type of program is more effective, religiosity etc, I think there are ethical reasons why comprehensive sex education should be the standard in this country.
The argument for abstinence only education is usually one of values, that in addition to preventing certain behaviors, the goal is to instill in them the "values" of our society, that sex before marriage is not accepted, etc. There are many problems with that, the first being that clearly, not everyone's values are the same. And on top of that, the values that the curriculum supports are more theoretical ideals than reality. Only around 5-10% of people remain abstinent until marriage. It's been that way since before the sexual revolution. Telling kids that it is not socially acceptable to do does not change that reality, and the discrepancy between what we preach and what we do does not go unnoticed.
I believe there is a moral argument to be made for comprehensive sex education. Ethical, and moral. The United States was founded on democracy. It is something we like to think we value. It is curious to note that sex education is one of the few subjects where the popular thinking is that less information = better choices. That the more the discussion is censored, the better things will be. When you separate the fact that this is education about sex, and the stigmatization that often goes with it, you are left with the guiding principle that not knowing something is better for you.
Accepting that, we have to ask ourselves whether that principle is valid. Whether it, not just the subject we apply it to, represents our values as a society. Is it not more democratic to give as much real information about a subject as possible, and then allowing those who receive it to make up their own minds as to how that information should be applied? It is not the schools place to instill personal values, it is the schools place to educate. Can we really expect someone to make an educated decision about something that can be as complicated as sex when they have received very little genuine education on the matter? Is the ability to make a healthy and educated decision not what we want from these kids?
Furthermore, if improving the standard of sexual hygiene in this country is a goal, as it should be, the issue NEEDS to be approached as just that. Hygiene. Healthful behaviors. Mentally, physically. STDs should not be presented as scare tactics, the oogey boogey of sex. They are, by and large, viral and bacterial infections that just happen to be spread by a mechanism that is highly stigmatized. This needs to stop. My university health center has to work extremely hard to promote sexual health among its students and change the perception about things like getting routinely tested for stds. You DO NOT have to be having "risky" sex to have an STD. Getting tested does not mean you are a dirty dirty whore. It means you are responsible. Medically speaking, even monogamous married couples SHOULD get tested. It's not a matter of not trusting your partner, having icky symptoms, having cheated yourself. It's being responsible with your health. It's washing your hands, coughing into your elbow instead of your hand, having an annual physical even when nothing seems wrong with you. It is not just sexual health. It is health. We need to start talking about it that way.