as Nettrice said, you have to accept responsibility for your choices. Part of that responsibility is making sure you don't scar someone else's life with genetic defects. Contraception is fine...

If you are asking if there is an absolute reason for the taboo, then no...if done with proper respect and responsibility, there is nothing that differentiates the relationship from anything else.

Is there any other reason for the taboo? I think it developed out of what I was saying earlier: family relationships are different in a way that generally skews the workings of a "normal" relationship. Just because it CAN be done right does not mean that it IS or that it is not taboo. The reality of the situation is that most incestual relationships are not done right.