Spoilers....

Normally I would feel bad for everyone in this situation including the exs current bf but here I dont...
Spoilers.
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I mean I understand that the exs current bf really likes this guy or whatever but saying the MC shouldn't have come back and just died???? Excuse me??? I'm sorry but he is getting annoying by blaming someone else for his insecurities. If his relationship with the ex was so shallow that when the MC came back the relationship was just going to shatter and you were going to feel super insecure then that's your fault. You shouldn't have started your relationship in such a way then. I dont think its okay to use the excuse "I love him so much".
So the secret the exs bf is keeping the ex is the fact that the ex was drunk/not in his right mind so he slept with the current bf may be thinking it was the MC or may be not. The next morning the ex thought he forced the current bf so he felt responsible for that so he gave him a bunch of attention. The current bf really like the fact that the ex started giving him attention so he didn't tell the ex that it was actually consensual....

I'm sorry (not sorry) but he is annoying. I get that he is feeling insecure but this is exactly why he is feeling insecure. The foundation of this relationship is built on a lie he kept.