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-!

From Nakama:

Kaibara Yousuke is a third-year college student looking for employment. During his second year, he'd immediately rebuffed his classmate Nakayamada Kazuki's offer: “Won't you have sex with me?” But that time, Kazuki's declaration of, “I could make you feel so good, you wouldn't be able to think of anything else,” was the beginning of their relationship. At first, Yousuke would shy away from sex, but he was healed by the unexpectedly tolerant Kazuki, and was gradually captivated.