I already refuted the biology argument in the original thread. Contraceptives exist.
I provided an explanation not an argument.
I explain one of the causes for social dislike of incest, I did not make an argument as to why incest is bad.
But granted biology certainly is one of the reasons commonly brought up, so I would understand your confusion.
If I had to make a counter argument then certainly the biological argument would be the weakest. No I'd rather focus on the affects on the individuals, the effects on the family, and then on society.
There seems to be few cases of individuals partaking in incest being mentally health and well adjusted, although one can argue that to partake in incest one already had to have something wrong since incest is a natural and instinctive dislike in people (self evidently so: the vast majority of people think incest is disgusting). So it can rather admittedly be argued that people who partake in incest have a history of sexual abuse, or (as is the case with Chris-Chan) have some diminished mental capacity, and so then the argument follows that they would have been screwed up regardless. It's a bit of a chicken or the egg issue, but I do feel that incest would have in and of itself have a negative impact individuals.
I hesitate to bring up the instances of an adult having sex with their child, since here you can make the argument that it's the paedophilia what causes the most damage. I'd however bring up that it is far more traumatising for a child to be raped by a family member (which is intended to be a psychological source of familiarity, safety and stability) than it would be to be raped by a stranger.
If we look at the purpose of a family it is to raise and nurture children and prepare them to one day become independent and then to form their own family. It is fundamental to society that we produce more people (literally a society will die out otherwise), and that these people be well adjusted and capable of operating in society. This is from the parent child perspective, but given that this conversation came out of Chris-Chan possible raping his mother, I think its a fine enough aspect to focus on.
Incest would be a fundamental shift in the familial relationship and I'd argue this shift would have a negative impact on the core goal of the family. So incest can lead to the breakdown of the family, produce maladjusted individuals and if seen on a wide enough scale can lead to wider scale social issues, and this is even if we ignore things like child molestation, or elder abuse, since seems almost inseparable from incest, because again normal people don't partake in incest.