Personally I don’t mind playboy Seme’s especially when most of it was before he met the main Uke. However the part that really was the worst was when he answered the phone while having sex with someone, continued to talk instead of hang up. And then afterwards he LIED and promised that he was just playing a prank with his friend on the phone. The lie and lack of transparency was the awful part. The main Uke already knows he’s slept around before but telling him such a lie and swearing it’s the truth is awful. He never told him the truth about that incident even to the end.
It’s all sunshine in rainbows cause they’ll keep it a fairy tale happy ending but reality would be that such a guy would only keep easily lying and his partner would eventually stop trusting him.
I don't agree with seme being "trash", he just enjoys free sex life, also he doesn't make those random ukes fall in love and ditch them as some of you suggest lmao. It's pretty clear he targets ukes with a similar mindset of "having fun". And he's definitely not cheating wtf they weren't in a relationship in the situation you're talking about. And after they got together seme never cheated and I'm pretty sure he won't in the characters' future, cus look at him, he's fully dedicated to his boyfriend. That manga is suuuper cute