The talk of the marriage was delayed as of now since Alex didn't want to take such an important step in his and their life, hastily.
He was not hesitant, not at all. The moment he decided to date them, he had already considered them as his life partners, so the marriage would be like tying an official knot to make them a married couple.
He didn't take them to church right away because his current priority was to take care of his soul. He had to finish the quest assigned by the system, and until that happened, he asked them to wait for him.
Naturally, Celestria and Sarah agreed since they also knew what was at stake. Their jealousy on one side, what all three of them mutually shared was the concern for Alex's life.
As such, Sarah and Celestria decided to get married once Alex stabilized his situation with the Curse and got better control of his Soul status.
....
