First, the uke was out here popping bussy for money. You can say it was due to shitty circumstances, but he was a prostitute probably servicing more then one guy at a time. Second, last time I checked the uke does know martial arts, so if this was really a no for him Makoto would have definitely made it known. He was definitely surprised, but only because it was unexpected. The fact that Makoto didn't scold the seme and even invited the young guy to stay in their house should tell you this wasn't a problem for him.
Following the story and characters, that scene was just suppose to be a spicy scene for us to enjoy. The morals of the real world do not apply to their environment and story plot. I'm not saying in other stories, depending on the plot, this scene wouldn't of been hard to swallow. Certain stories are built to test morals by reflecting real world scenarios, but this aint one of them. I understand not everyone is a fan of voyeurism, and that's cool. Just go find something more wholesome to read, instead of littering the comments with all your negative comments. Lol