As a re reader, I want to clear your misunderstandings. But it sounds like a spoiler, so I'll use spoiler option to explain it.
Spoiler!
ML doesn't believe single thing MC said. But ML want to avoid his marriage with his step sister and marrying this guy is a better excuse to avoid that marriage. On the other hand, MC doesn't want to use abortion and MC want to keep the child. So marrying each other sounds like a win-win situation.
.
Omegaverse >ω< https://mangamirror.com/manga-list/106967
.
Historical BL ♛➛ https://mangamirror.com/manga-list/122136
.
Best stories ⛧⁀ https://mangamirror.com/manga-list/106911
❤️
