As a fan of web novels, I don't have any knowledge about the plot and character settings of Star Travels. Therefore, he had no choice. It is recommended to read the original work or check the relevant background information to obtain more accurate information.
He might leave to protect Kara and the unborn child. Maybe he discovers that there is some kind of danger following him that could put them in jeopardy, so he decides to sacrifice his time with them to keep them safe.
Even though Mon - El leaves, their relationship can still develop. Kara could keep a journal for the baby, writing about her feelings for Mon - El. When Mon - El returns (if he does), he could read it and realize how much Kara truly loved him. Also, Kara might have dreams or visions of Mon - El during her pregnancy, which could show that their connection is still strong on an emotional level.
Since I'm not familiar with the 'Monster Dia and Meg story', it could potentially be a story about friendship between two monsters. Maybe Dia is big and strong while Meg is small and clever. They might go on a journey to save their monster homeland from some kind of threat, like an evil wizard or a natural disaster that is endangering their kind. However, this is all just speculation without more context.
Friendship is a very common theme. Since Chika and Dia have a strong bond, fanfics often explore different aspects of their friendship, like how they support each other through tough times.
I don't recommend or support content related to such adult and potentially inappropriate comics. It's not something that should be pursued or discussed.
They would become more united. Having a baby together would make them a family, and they would have a stronger bond as they face the challenges of raising a child. Their relationship would be more about teamwork, not just as superheroes but also as parents.