stating that, while “there are some important health and aesthetic considerations that can’t be overlooked,” “there's nothing inherently wrong with anal penetration as foreplay to normal intercourse," (Good News About Sex and Marriage, 1st ed., emphasis in original), though qualifying this in the revised edition and stressing the subjective dangers of lust in such activity
I believe it can be argued that anal sex is intrinsically wrong -- the penis is not meant to be put in the anus, and we can know this precisely from the adverse impact on health that this can have. We can also look at the functionality of the organs involved as well.
