The uke didn't want to marry the 1st guy he was engaged to apparently he is abusive trash. So he lied and said he got pregnant from a one night stand to force his family to break up the engagement. && Since everyone thinks he is pregnant the family allowed him to marry the Seme. The Seme is going along with it because he feels bad that he sleep with the uke during his rut. So the Seme feels like he kinda forced/raped the uke (but he didn't).So he feels guilty and then on top of that he has some past trauma dealing with another omega and apparently he made some vow about not having sex before marriage and since he did and broke it he also feels like he needs to marry the guy to someway fulfill the vow.
Oh forgot to add since the uke lied about him being pregnant and the Seme agreed to go along with it. They have to produce a baby so the families doesn't know it was a lie and whatever business dealings the uke and his family had with his former fiancee won't get thrown away. So kinda like I know my son got caught up and someone else rut and he got pregnant so it's not our fault so you can't end the business deal since it was and unfortunate event.
why do they want a kid again? i completely forgot the plot