 If you liked
Injuu Gakuen La Blue Girl
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 ...then you might like
Ninpou Midare Karakuri
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Both series have a plot with ninjas and sex. La Blue Girl is more hardcore with tentacles, demons, rape and so on. Both series have a "old school" anime style to it.
Ninpou Midare Karakuri is a hentai/softporn which focuses on vanilla sex, public sex and comedy. It has the normal boyxgirl thing. With a little twist of ninja spells, potions and threesome to add some flavour to the sex.