Certainly the biologically implications of reproduction are a concerned; but reproduction is not a requirement for marriage. If it is the reason, then it would be prudent to apply the same restrictions on everybody by requiring genetic tests for marriage.

Please, if you have a good argument against it, I am willing to hear it.

…because?

It becomes even more ethically “interesting” when you consider assisted reproductive techniques.

In most countries sperm donation is still anonymous – and while there is generally legislation limiting the number of offspring a donor can father, there is a chance that half siblings may marry unknowingly.

Then you have cases like Dr Jacobson (http://query.nytimes.com/gst/fullpage.html?sec=health&res=9E0CE0DE1F3BF93AA35756C0A964958260)
who fathered over 80 children, most through using his own semen in sperm donor cases.

I remember reading about one such case here in New Zealand, where the couple involved were harassed by the Police on a daily basis until they ended up fleeing their home town and changing their names.

These cases support the imprinting theory (or at least doesn’t contradict it), as those involved wouldn’t have had the opportunity to know each other as siblings as children.

On an unrelated note, the issue of marrying a sibling is one of those which highlights gross inconsistency in the Old Testament; see here for details.