
Idk about you guys, but I can't really consider it as "incest". Ignoring the science behind why incest is wrong (since this is bl, they can't have children), incest is a taboo because of social reasons.
In this case, neither of them knew that they're related. They did feel disturbed when they learned the truth, especially the father.
And in the end, the father did attempt to stop what happened, although if he did, he'll change the past/present/future, leading to him losing his family. That's why he didn't. I loved the ending where the father declares his love for his child with "you're my precious child" and didn't end as full-blown incest.
OMG. AN UPDATE AFTER THREE YEARS. THANK YOU VERY MUCH! YOU ARE ALL HEAVEN-SENT