Rosario to Vampire, Sora no Otoshimono Recommendations
both males have a new type of species introduced to them
both are harems with plenty of girls
at first both of them wish all that happened didnt happen, but they start to enjoy it and like their new lifestyle
Echhi, Harem and, Romance. Need i say more?
Alright, so both of these series have supernatural attractive female beings from a different realm (in a technical way to put it)
The romance isn't as heavy set as Rosario to Vampire (in my opinion), but it still gives you that on edge feeling about whats to happen next, not to mention that Heaven's Lost Property is pretty fucking funny, I died in laughter throughout the entire series.
Try it and see how you like it.
Both are about a boy who without warning is thrown into a life surrounded by girls from another world.