There are many types of human sexual behaviour which are not "tolerated"
Incest is banned on health issues, but the health problems associated with homosexuality are far greater than those associated with incest.

Sexual relations between close relatives are common in nature....much more common than same gender sex, and if the means of procreation were removed or blocked, I suppose the behaviour would be completely safefrom the standpoint of physical health.

Personally, I am no more in favour of the promotion of incest, than I am in favour of the promotion of homosexuality.
This is simply another example of the law being an ass.