I completely disagree with this sentiment. I don't believe it was Meiko's intention to break the two up, I believe…
Meiko still went behind Makki's back and slept with Eiji and then lied to him that he was his child. She was a horrible horrible friend and a homewrecker. She couldn't have done that if Eiji wasn't a spineless coward and a cheater too. But in the end she was half responsible for the betrayal. Although she thought she wanted to help them, as soon as she hot pregnant she saw the baby as hers and she successfully and permanently latched herself to their relationship. Deep down there's no way she didn't realize her behavior was destructive and still went ahead with all of it.
I've watched a lot of japanese BL series and movies before this same with sad ending or have a ending that portrays…
They know the father is Eiji because of the age of the fetus. It showed that they Eiji and Mieko had sex alone behind Makki's back weeks after that and that's when she got pregnant, then they lied to him and said she got pregnant when they had the threesome.
Its good there is no happy end. They made decisions and we see the consequenze of it. Ejii has a child he loves…
He couldn't bear it because they continued to have sex behind his back and lied to him. Of course he figured out he need needed to get out of that situation.