I have not researched it, but I have heard of a few rare cases of real accidental pregnancies. A guy was taking a shower and his semen got on a washcloth (let's not go into how

). A girl took a shower directly after him and used the washcloth to scrub her

, and the little swimmers took advantage of the situation. At that point, it is completely in the hands of the woman. If she did not ask him to sign the birth certificate, then later on she'd have a difficult time even forcing him to have a DNA test. But if she did ask him to sign the birth certificate, and he refused, she can file a paternity suit, and he's pretty much legally bound for the next 18 years, unless he skates off to another state far away.
Even in that situation, it is pretty tough to argue it was 100% accidental. Unintentional? Absolutely. But getting sperm on something that has a good possibility of getting near a woman's

is not the smartest thing to do.
The OP should change the topic title, since we've pretty much established that sex is not an accident. And in the very very rare case it was accidental, the man is still at the mercy of the woman as far as child support goes.