Rosario to Vampire, Monster Musume no Iru Nichijou Recommendations
A harem involving a generic looking human boy surrounded by love interests that are....girls...youkai monsters that is...the only difference is that the human-youkai co-habitation doesn't exist and is shun from the human world. Also the youkais are supposed to blend in human society instead of unleashing their true youkai form in the public.
Every monster girl the MC encountered, everyone falls in love with him. Rosario to Vampire has the highschool setting and MMnIN has more of the family bond setting. The two anime has mythical "human creatures" and the MC is just a plain human bait, but they're sincere. Definitely ecchi for both shows.
-Both series are about a human boy who interacts with monster girls.
-Both main male characters have a harem with the monster girls.
-Both series have comedy and are extremely ecchi.
Both are Ecchi Harem Anime about a human boy being loved by multiple monster girls. While Monster Musume takes place in the human world, Rosario + Vampires setting is a school for monsters.
Both male MCs have a major problem: Kurusu is not allowed to have sex with the girls, Tsukune must hide the fact that he is a human.
-Both anime tell about everyday life of a boy surrounded by a non-human girl.
-Both anime have same genre, especially harem, ecchi, romance, comedy, and fantasy.
-And of course, both anime contain many fanservices scene, better watch the anime alone.
Both have a main male character that is normal but one day gets himself into a life full of monster girls... There is where their adventure begins
If you're into non-human harem, then this is it. You've found it, congrats. However, be aware of the differences : Rosario Vampire is more serious and "school life" while Monster Musume is entirely focused around ecchi and comedy.
Both are ecchi/harem anime where the girls are monster girls of various sorts. Also in both cases, even though the girls are friendly toward the primary male protagonist, their monster nature sometimes gives rise to problems or awkward situations, both for him and with each other.
Both series are a harem of a mortal man surrounded by monster/supernatural girls who love him and their shenanigans.