Do you think people who masturbate to rape porn want to commit rape? Do you think people who masturbate to incest porn want to commit incest? These are the two most common fetishes in the world- one trip to PornHub can tell you that.
First of all, how common something is has
nothing at all to do with the morality of the subject. Murder is common, but you're not going to convince sane people that it's okay on the basis of its occurrence rate.
Regarding rape fantasy, I think it might indicate at least less negative feelings about rape. They
may not want to commit rape, but they also may not think it's bad either. To put it another way, if I were a woman and I found out some man had a rape fetish I'd be careful to avoid them, and rightly so.
As for incest, I think it depends on what type it is. The only type which cannot possibly indicate a willingness to commit incest is a variety you
cannot personally commit. For example, everybody has a mother, and if a man is into mother/son they're almost certainly harboring incestuous feelings.
However, if he's into lesbian sisters and does not even have sisters, then, while that is degenerate, it can't indicate a personal willingness to engage in said behavior because he's not a female with a sister.
From another angle, it'd be like saying a man is homosexual for finding lesbian porn erotic. It's not indicative of a willingness to participate in homosexual activity on a personal level, so it could be seen similarly in some cases.
While true, tiny adult women exist, and not just in anime (where it feels like in some cases, in ecchi especially, they give them explicit legal ages just to justify drawing loli bodies). So is it wrong to ogle a 20-year-old woman if she has the physique of a 12-year-old?
I'm not sure that's really possible though; if you watch To Catch a Predator you'll see that even the youngest looking women they find have to only be caught a glimpse of before the predator enters the home, otherwise they might catch on. Often it's just their voice from behind a door or positioning themselves in a way which obscures full view, and they run away out of view back into the home (if they're visible at all) once the predator approaches.
I go back and forth on hentai. On the one hand it's just lines on paper no matter what is drawn, but on the other hand I do think intent matters. I just bypass the issue and advocate for the banning of pornography entirely. These discussions only reinforce that feeling, look at all this degenerate shit, the world would be better off without any of it.