Hmmm this manga has a similar plot with kowarekake no omocha where uke-san came from a poor family (in this case, his parents were deceased) and was bought by a rich family household to be their son's playmate/companion and when they're both grown up seme-kun treats uke-san as his sex-slave, although I think seme-kun (akihito) is not as forceful as Kengo-san. Both of them never show that they're interested in girls, moreover seme-kun often jealous when uke-san befriended with other guy.
Saihate no Kimi e