The current mess of consent laws in the US stems from the varied success of turn of the century morality pushes. The initial arguments were geared toward protecting innocent young women from the predations of older men. Then, as now, legal solutions to social phenomena were largely prompted by prejudice and an overwhelming need to get involved in someone else's business.
predations

There are probably 12 year olds who do want to have sex. Some of them, I'm sure, will want to or will actually have sex with older people. I don't see a problem with that situation, as long as both participants are willing. The problem comes in when coercion and force are involved. Neither coercion nor force should be tolerated in any relationship, much less a sexual one. Fortunately, pretty much all societies have both laws and social repercussions to deal with those situations. Currently, I think the US sometimes goes too far.

We now have sexual harassment laws that put a big damper on people meeting at work and starting a relationship, partly because many people view the balance of power to be problematic (possibility of coercion). I have quite a few friends whose parents would never have met under these circumstances since they met at work and even flirting is discouraged at many workplaces. We're now seeing a huge surge in social networking solutions to the problem created by it being socially unacceptable to find someone to date at work, where people spend an increasing amount of their waking hours.

My parents met when my mother was 16 and my father was 24. California law now, and then I believe, would have treated their relationship as statutory rape. My two younger sisters and I would not exist if my parents had had to deal with the same social conditions as now. Nor would another one of my friends, who is also the oldest of three, and whose father is 10 years older than his mother. My first few sexual experiences were technically illegal, since I was underage and doing it with people who were much older than I. The current attitude, that only people 18 years old or older are responsible enough to have sex, is frankly ridiculous in my opinion.

When you get into the position of legislating morality, you often get into the difficulty of judging where to draw the line. In debates about this very subject, the age of consent, you rarely see a consensus. Some people even think the age should be set higher than when they themselves first had sex. I think that it's actually much easier to figure this out on a case by case basis since the cases are self-screening. If there's a complaint, there exists a possible case of coercion or force being used. Both force and coercion are much easier to objectively determine than the age at which everyone should be able to have sex. Most rape laws would work for charging a perpetrator regardless of the age of the victim; consent laws are almost superfluous.