Morally? Not really, two consenting adults, regardless of relation remove the moral aspect of the act. However, as has been pointed out repeatedly, it's the consequences that are very morally charged.
For the pair, the act (or indeed relationship) would need to be hidden, either fully or through some means of deception to appear publicly as a couple due to the moral and social ramifications. If that's as far as it goes then I can't really blame anyone if that's the road they choose, but discovery could do massive damage not only to themselves but to their family and friends.
I also believe it's an unhealthy misconstrual of familial love for erotic love and creates a slippery slope from a societal point of view. When a child comes into the mix, however, it's a whole different ball game.
On a purely genetic level a first generation child from incestuous relations should only have a marginally increased risk of abnormality. So from the perspective of health it's not a total disaster. But that's where the relative upsides end.
Could you ever bring that child to meet the family? Could you hide the truth from your child, knowing that they may unwittingly reveal your secret by discovering their schoolmates have two sets of grandparents? If you were to have a second child, could you deny their own possible incestuous relationship, as while your grandchildren from this arrangement run a vastly increased risk of disorders, you are something of a hypocrite for closing a door you opened.
Even if you could find a workable solution to these adversities, the laws are in place for a reason. Institutionalized incest could cripple a society within two or three generations, yet our concern for the rights of the individual (which I wholeheartedly support) clouds our ability to decry people who choose this path.
I think you have to outlaw it on the grounds that of course, it will still occur, but the illegality will dissuade the vast majority (although many other factors would be more prominent in preventing incest than the legality) and therefore I can see my way to having no moral objection to a childless incestuous couple, but that is my line. However, the concept in legal terms is unenforceable, plus the issue is not so widespread as to require such legal alterations...