This is also a weird case, because the problem with age difference is maturity, which is not measurable as of now. A 30 years old dating a 50 years old is weird, but less than a 20 years old dating a 40 years old, and yet all of these also can vary based on individuals. And most people wouldn't see a problem with a 70 years old going with a 90 years old. That's why it's struggle to make well-defined laws about it.

For incest, once again, the problems are the norm, and then maybe you can find some exceptions. But why push it to be fine and allowed, when it doesn't bring anything, again? It's just opening for more issues to come, with no real benefit because incest is, in most cases rather than not, a problem.

And how do you prevent problems of offsprings, or of dominant position? Do you have to sterilize by force people in such relationships, after having them investigated to check that the relationship is actually consensual? Who would need to have sex with their relative so much that they would want to go through all this? That's why I'm saying that it just feels like a lot of trouble and problems, for nothing good.