Him using him as an emotional punching bag was only so he could guilt Youhei into staying with him. Youhei wouldn't have stayed with him otherwise out of self hatred and his guilt towards the seme for the incident. Youhei definitely wouldn't have dared let the seme take care of him in any way and the seme knew that so he used a nasty method of making sure he wouldn't leave his side so that he wouldn't get physically abused by his family. He was desperate to find a way to bind him into a safer environment for him for selfish and selfless reasons at the same time. It was an ends to a means. Honestly the only reason Youhei was able to move past his guilt and profess his feeling and start to heal was because he was able to process things in a safe(to him it was safe) environment. Youhei was not unable to smile because of the seme, it was because of the guilt he cast on himself due to his self loathing, If you notice, he was able to smile by himself after coming to his own self-realization and being able to express his love to the seme.
I hate how the seme treats youhei like shit in the first part of ch3. It's one thing to bind him (he didn't know how Youhei actually felt, so in his pov he bind Youhei) and another to make him your emotional punching bag. Wanna know why Youhei doesn't smile, even just faintly, around you? That's because you treat him and the situation you trapped him in like shit.
I even still feel like shit even after the warm extra.