But the dividing line for prohibiting immediate family incest is pretty damn clear...
If a married woman conceives a child through in vitro fertilization, donor eggs and her husband's sperm, and later has sex with that child after the child is grown, is it pretty damned clear that that act is illegal incest?
I think the point where I am inclined to disagree with you is not about the desirability of eliminating incestuous relationships. I think incest is pretty creepy, and minimizing it is indeed a benefit. The question is that cost and benefit relationship you keep talking about. What are the real benefits and what are the real costs?
For example, I have known one couple that I am extremely confident was engaging in an incestuous relationship. They were both over 18 at the time. They were raised together as siblings. They had the same mother and different biological fathers. I considered them fairly bizarre people, especially the male, before I realized they were sleeping together. The fact that they were sleeping together convinced me that there was something seriously wrong there.
So, with such a couple in mind, what are the costs and benefits of anti-incest law? Although they were not deterred from their relationship, others probably were, so it cuts down on the number of such relationships, probably. That's a benefit. It forced them to be clandestine about their affair. That's a cost if you ask me. Any time people are forced to lie about their relationships, it creates stress. That's bad. Furthermore, it actually might create a situation where one member of the couple has leverage over the other, blackmailing them or some such. That's bad. It also cuts off the sort of normal social support network that people in unhealthy relationships usually have access to. Let's be real. When chatting with her girlfriends about their lovers, she couldn't say, "I'm not sure if I should stick with my brother...." That sort of support network really is important and a very useful aid to people in deciding whether or not to continue relationships.
Meanwhile, if their relationship had been discovered, they could have gone to jail. That's a very high cost, wouldn't you agree? As bad as their situation might have been, prison would have been much, much, worse. Of course, prosecutors might decide to use "prosecutorial discretion" for one or both, but really that's barely different from "selective prosecution", which gets convictions thrown out.
Finally, the existence of the law establishes a precedent that says that we can criminalize consensual sex of any form if we show that there is some benefit.
So, we add up the benefits: Fewer incestuous relationships, versus the costs, which consists of forcing people into the shadows in their relationships, plus establishing a precedent for government control.
It's not absolutely clear where I would come down on weighing costs versus benefits, but in this case I lean toward the "it's none of the government's business" side of the equation.