The reason Dave is that it is believed incest would upset the balance of society.....the norm was heterosexual marriage and since heterosexual marriage has gone into decline, so has society there are now huge numbers of single parent families....causing bewildered and insecure children and additional cost to the benefits agencies.

But, although heterosexual marriage rates are falling and homosexual "marriage" and union rates are rising a little, only amongst MSM are STD infection rates increasing.
Does that point to Homosexual "marriage" as a cure, or even a control on infection rates......Doesn't look like it.