Rentaro can't keep up with his utilitarian ethics on maintaining Enju's happiness and hiding the truth from Enju. He'll eventually have put his utilitarian ethics (like he did during the Fugitive arch) and his relationship to Enju to the test and have to tell Enju the truth about the fate of most cursed children and her corrosion rate. How Enju will react will depend on how she will find out. But remember that Enju happens to be one of the most kind-hearted characters in the series who is willing to forgive everyone.
Considering the left-leaning nature of the series, it's most likely that a religious fanatic group who wants to call for a genocide against cursed children will be introduced as showing the evils of religion in regards to Fantastic Racism. Bonus points if Enju and Tina gets kidnapped by the said group and the said group rapes Tina and Enju.
Since Seitenshi mostly serves as a Martin Luther King Jr. type figure towards the cursed children, it'll make sense that the series will introduce a Malcolm X type Anti-Hero who shares the same goals as Seitenshi. However, unlike Seitenshi who wants to change society through peaceful means, the Anti-Hero is willing to resort to violence and terrorism to achieve this goal. He will most likely will lead a terrorist group (mostly a communist group) consisting of cursed children and former Promotors/Initiators who are angry on human society treating the cursed children as trash and feel that Violence Is the Only Option to achieve equal rights for the cursed children. The Anti-Hero will try to convince Rentaro and Kisara to join his cause, but as long Enju and Tina are around, they'll make sure that Rentaro and Kisara doesn't join the terrorist group.
At worst, Enju and Tina gets killed off and Rentaro takes his utilitarianism to extreme and the world will end up being worse. At best, it'll be a Bittersweet Ending, with either Enju and Tina surviving and completely immune from turning into the Gastrea, or some Heroic Sacrifice is involved.
All three of these villains will have some grudge against Rentaro or want to ruin Rentaro's life. They'll put Rentaro in a Sadistic Choice situation where they kidnap Enju and Tina, tie them up, and have Rentaro make a choice: either save millions of lives in some faraway city or save Enju and Tina. And they'll make sure either choice will make Rentaro fail. If he tries to rescue Enju and Tina, he'll find out there are different cursed children there and Enju and Tina is revealed to be somewhere else and killed off. If he tries to save the city, Enju and Tina are not only killed off, but the bomb will activate and kill everyone in the city. In other words, The Bad Guy Wins and Rentaro enters his path of darkness.
If anything bad happens to Enju or Tina, this will be enough for Rentaro to enter his path of darkness and take his utilitarianism to the extreme.
The light novels stated that all cursed children are female. That being said, it's possible that a cloning story arch will be involved and male cursed children will be introduced (who happen to be clones of the cursed children). It'll turn out that Enju and Tina have "brothers" who happen to be clones of themselves. The clones are made to replinish the loses in the battle against the Gastrea and the clones are meant to be stronger than the cursed children. Not to mention, unlike the cursed children, the clones don't turn into the Gastrea at all.