Umbilical Sex: Ijou Henai Shoukougun

"I'm not gay." Makoto confesses his feelings for Kei, a playboy who goes out drinking almost every night. But Kei publically humiliates Makoto by rejecting him in front of all their friends. One night, while Kei is at a massage parlor, he gets a prostate massage, and his life changes forever. Soon, he starts experimenting by himself, and gets hooked. This makes Kei question his sexual orientation.

(Source: Renta!)