It's sexually immoral. Why? The Bible says no (Leviticus 18:6-18). As a Christian I try to follow it.
Should it be legal? Maybe, it's a hard question to answer. If both people say it's okay and they feel right about it then they should be allowed to do what they want. It's just another government control.
So, because the Bible says so, it's wrong? That's a terrible attempt at justification. The Bible in essence isn't meant to be taken literally whatsoever. It was a set of laws, morals, and judgements to live by. To make yourself a better person in the long run.
Why is it immoral, darth rave? Because you have the same genes?
Well if you believe 100% in the bible, then technically this is all incest. Adam and Eve were the first humans in existence, correct? Well they had sex, made a baby or two, then had sex with those babies and the babies had sex with eachother and thus, the life cycle of the human being was started.
Don't bring the bible into this, because it's a big ass book of deceit and contradiction.
On a related note, I think incest is right as long as it's not for procreation. Who cares if you have a *really* hot 2nd cousin and you two wanna bone?
Perhaps we should also stop stupid people from breeding. And short people. And skinny people. And bald people. And people without blue eyes and blonde hair.
Contraception consistently fails, especially condoms.