Incest among adults is also sex between consenting adults.

True, but that's largely a noncentral fallacy.

Most arguments against incest are arguments where the bottom line is already written since they are made by people who just don't want to admit they are plain grossed out by it.

If I'm grossed out by it, why am I watching lesbian incest hentai? :-)

Not only the motivation, but many of the arguments are basically the same as arguments in favour of homophobia. If the person's has an identity centred on fighting "bigotry" cognitive dissonance hilarity ensues.

Agreed. But not all the arguments are basically the same. Some of the arguments are more like the "teachers shouldn't date their students" argument and the "psychologists shouldn't date their patients" or even "50-year olds shouldn't date 20-year olds" argument, and reflect on the likely unhealthy effects of power-imbalance.

The power-imbalance in intergenerational incest is obvious. In intragenerational incest it can of course be significantly less clear; especially in cases like the German couple where the siblings only met during adulthood.

Bonus round: Arguments against incestuous couples having children is a fundamentally eugenicist argument.

That's a plus to those arguments, not a minus -- because we're moving to a consequentialist perspective from an arbitrary deontologist one.

Applying it like a consequentalist results in concluding many other kinds of couples should be discouraged just as much

Perhaps they should -- but keep in mind that banning incest bars any one person from a very small subset of potentially desirable sex partners -- much like barring psychiatrists from sexing their patients. On the other hand barring e.g. old people from having sex, or gay people having sex, pretty much precludes them from having pleasurable sex altogether. The cost of such a policy seems higher in such a case.

That having been said, I'd have been all in favour of applauding the German couple (especially since they didn't grow up together) if they had only made sure they didn't have children via e.g. vasectomy, getting tubes tied, etc...