The ML doesn't believe the MC anyway because of what happened when he called the ex after the MC announced the pregnancy. The ex got angry because, at that time, the MC had refused to sleep with him out of love for the ML. However, the ex didn't admit they never slept together; instead, he suspiciously claimed that if the child were his, he would take responsibility.
Reading this, I felt like the ex is also trash. He is the primary reason the ML misunderstands the MC. I know the MC did some shitty things to the ex, too, but still. The ML is also terrible; after they had sex, he claimed he had a fiancee, but then he suddenly offered to marry the MC even though he wasn't sure the baby was actually his. As the comments said, the ML won't lose anything in the end. Plus, he never even wanted to marry his arranged fiancee in the first place. He’s basically treating the MC’s feelings like a toy.

After finding out he’s fertile, he accepts the MC, but the ML isn’t completely sure the baby is his because MC’s ex told him they slept together during that time.
So why did the ML accept the MC anyway? At first, it was mostly for his own benefit. He wanted to get married to gain a large percentage of shares and then get a divorce later if the baby wasn’t his.
While preparing for the wedding, they start living together. During that time, the ML notices how hard the pregnancy is for the MC and starts taking care of him. He gets worried, especially because the MC isn’t eating well.
The MC, on the other hand, feels like he has to be very careful all the time. He doesn’t want to disappoint the ML, so he hides his feelings.
After they get married, they sleep together again, and things seem fine. But one day, the MC goes to the ML’s office and accidentally overhears him talking with his best friend about the divorce, what he plans to gain from the marriage, and how he still doubts the baby is his—even though he told the MC before that he believed him.
After that, the ML is the one who asks for a divorce and lies, saying he’s not the father. That’s when the MC begs him not to and confesses that he loves him.
The ML doesn’t believe him. Suddenly, the MC feels strong pain, and at the hospital, the doctors say he might have a premature birth.
While the MC is still in the hospital, the ML starts questioning everything and wonders why the MC kept insisting he was the father. He decides to meet the ex, who tells him they never actually slept together. That’s when the ML realizes the baby is his and understands he was wrong all along.
Even then, the MC is still hurt and wants a divorce. After giving birth and once the baby is safe, the MC decides to live alone with the baby.
The ML apologizes and says he will only come back when the MC is ready.
Later, when the baby gets sick, the MC has to call the ML. After that, they start talking again and slowly get closer.
In the end, the MC forgives him, and the ML moves back in with the MC and their baby.