KYOKAN HUNTER

From Nakama:

A phantom thief has been creating an uproar, passing through every defense and stealing countless works of art! When he takes a liking to the "maidenhood" a guard sacrifices to him, he leaves without stealing the goods... The guard of a certain art museum is completely taken in by that scheme. Their relationship began with contractual sex but before they know it, the two of them are attracted to each other.
However, the thief's younger brother kidnaps the guard to get revenge on his elder brother and a big group gang rapes him-!