Choukui no Hana

A police detective is sent to investigate several mysterious deaths involving men who die while having sex. He encounters an extremely beautiful police chief who is specially assigned to investigate the serial murders. As they dig deeper into the case, the detective discovers the existence of two special "type" of humans who have special / hidden physical stats and abilities, namely the "Flower" clan and the "Butterfly" clan. What has it got to do with the deaths? [Edited from Aarinfantasy Forums]