The problem arises when you take into account that lust is ultimately within the control of the viewer. You also have to take into consideration intent and context.
Now I’ve seen some ecchi manga that’s just over the top. On the other hand, I’ve seen just as many that know how to deploy the themes well (be it in the form of comedy or realism).
You’d have to be careful in making statements like this as they may work against you (especially when your opponent is aware of, for instance, the nude Renaissance paintings found in the Vatican museums).
Agreed that ultimately, hentai reduces your mind to that of a sex-crazed animal much like porn. However, unless you make that the ONLY reason for why they are both equal sins, you’re going to have trouble arguing with people who insist that one is not as bad as the other.
I have to admit, one can say that real porn is of a ‘worse degree’ since it exploits more people, involves real sex acts, and arguably has a wider grip on the population. Hentai doesn’t really have a strong foothold in that granted that it uses completely fictional characters, fictional bodies, and some of the disturbing things I’ve seen look too revolting to even be possible.
However, such an argument is moot in the fact that it still reduces your mind to lust. It was built for that. In fact, the 1% of hentai that I can’t criticize for being tasteless trash are in fact hentai that can actually do away with the sex. In other words, they are hentai that didn’t have to be hentai in the first place.
One big example: Fate/Stay Night. It started as a hentai game but it managed to move on to a franchise that no longer requires too much graphic displays of sex. It’s been granted a clean version without any serious effects on the plot.