I never said people fail to use them all the time. Contraceptives are there for anyone responsible enough to use them. If they don't, then that's their problem. Now here are the points I'm trying to make.
You did. You said that not all the people will use them all the time. That effectively kills your previous argument. As for the rest, I will quickly address them as I am growing weary of this discussion.
1. There are no laws or regulations about people at risk for passing on birth defects or genetic diseases. The only time people make a big deal is when the risk comes from relatives.
Not true. Laws, mores and folkways exist regarding individuals with transmittable diseases, mental and physical handicaps.
2. Contraceptives are available for anyone with sense. People being irresponsible enough not to use them is a different problem altoghether.
No, it is the same problem, unless incestuous couples are going to be somehow magically incapable of irresponsible behavior.
3. No one pointed out a way that this hurts a third party.
Everyone here has, and since page one. Offspring.
4. With relatives close in age, there is no inherent imbalance of power or dependency.
There are plenty of same-age relationships that have an imbalance of power and dependency. Look at the volume of abused spouses. Are incestuous couples somehow above this as well?
Look, it just doesn't work on any level. You seem like a nice guy and I don't think you are trolling, but putting your fingers in your ears and telling us that you don't hear us is kind of silly.