The Rapeman

Batman: Gotham Knight is not hentai (well no duh), but these two anime have a similar dark feeling to them. They both have a superhero that patrols a dark and corrupt city with their own view on justice. They have enough action to keep you entertained. 

Both are 90's hentai about male protagonist who gets paid to have sex with women. Main difference being one of them does it consensually while the other one doesn't (take a wild guess which one). 

Both have absurd, though Rapeman has more serious in tone. Both work for JUSTICE!!! Inferno Cop is totally non-hentai stuff (well except for drowning in a pregnant women's 'waters') 

Although they are different genres (with Rapeman being a hentai), both present the superhero genre in a unique way. The main characters of both anime are masked vigilante who patrol a dark and corrupt city, also dealing their own brand of justice. Also, both main characters have alter egos that have respectable day jobs. 

They are both hentai in nature and have comedy though Rapeman is much funnier.  

Both anime cover serious material and though some watchers may be thoroughly disgusted some people (like me) will laugh a lot watching. Also in BNP it's yaoi so just pretend it's futa man and you'll get through it without being too mentally scarred.  

Both series have beautiful women and lots of rape.