Maybe because song was legit the reason he got bullied and was an outcast for so long at his school
Like if song treated him well at school mu wouldnt even need to have an "andrew" cause the sole reason he even became buddy with "andrew" in the first place is because he was lonely and outcasted
I understand that he is hurt I really do but he had it coming and he is an adult, actions have consequences ┑(￣Д ￣)┍ Mumu was not refusing him just because, he was scared too because he thought Song hated him and he can't read minds, you naturally won't give a chance to someone you don't like or don't feel comfortable around to see that maybe they are not that bad...no, you will run away from them ans then again, Mumu did not liked him, he wasn't attracted to him and that's valid too.
Mumu did nothing wrong, I feel empathy for Song's situation but he had it coming.
try to understand also the seme, he was hurt by uke's reaction, after all he was not playing with him, the seme also wanted to approach uke as himself, but uke refused him, so he had to keep playing pretending to be "Andrew", the fact that he hates the seme and doesn't hate "Andrew" hurts me too not only the seme